Third and hopefully final draft. Really just double-checked and touched up a couple things.

Masamune, the Black Tortoise King
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Age: At least 1700, estimated to be over 2000 or more
Gender: Male
Height: Usually 5"6"/168cm (slightly hunched posture); sometimes 6'/183cm
Weight: Usually 120lbs/54.5kg; sometimes 220lbs/100kg
Station: Black Tortoise King, Inventor of Zanpakuto, Forger of Asauchi, Royal Smith
Reiatsu: Warm water flowing over you; sometimes in a gentle stream, sometimes in a raging torrent.
Speech: Maroon

Description:
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Masamune is normally a short, hunched-over old man with a modest-length white beard and mustache, a bald head, clear green eyes, and a rather wiry look about him. When meeting with the other Royal Guard, he wears a traditional-looking Shihakushō. When working, he often is clad in nondescript but robust clothing and a leather blacksmith's apron and gloves. Often as much either way, he's wearing darkened goggles one might wear when welding metal. Either way, his feet are typically shod in heavy, metal-toed boots.
When he's not in an official meeting or actively working, or in a meeting but doesn't feel like dressing up, he wears....cargo shorts and a tropical shirt, with sandals on his feet and a fruity and likely alcoholic drink in his hand. Because that's what he feels like wearing.
All of these outfits are actually the Shihakushō that the Azure Empress-Dowager crafted for him; between her instruction and his own knowledge and skills, he's simply able to make it change shape more often and more widely than most. Because he likes wearing those clothes, dang it!
Of course, all of this is if he's "at rest" or otherwise working normally in his workshop. When he releases his Shikai, or brings up a large amount of his spiritual power, his body actually increases in height, and goes from "kinda wrinkly, wiry old man" to being in incredible shape. He gains several inches of height, and almost a hundred pounds more mass, much of it muscle. All of the hunched posture is gone, and he stand straight and tall.
When he unleashes his Bankai, all traces of age disappear as his full power courses through his body. His hair goes black, his beard grows to the point of reaching his belly, he gains a full head of wild hair that reaches his shoulders, and all wrinkles disappear. His overall countenance as a "young man" is much fiercer. Considering how rarely he looks this way, and what it takes for him to be in this form, it's no wonder there's a hard and fell look to him when in this state.


Personality:
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When meeting in an official capacity, Masamune is polite and well-mannered. He finds his comrades to be "too tense" at times, and has a tendency to prefer to "go with the flow" more often than not. Certainly he finds the Red Pheasant a bit...uptight...at times. Still, he has a strong sense of duty, tradition, and brotherhood, and firmly believes in the laws, rules, and traditions of the Soul King, Royal Guard, and Soul Society.
When he's working the Forge, or trying to teach someone to work it, Masamune becomes a grueling taskmaster, demanding careful work and perfectionism. He constantly lives by the mandate that imperfections in the work of forging Asauchi kills Shinigami, and thus cannot be tolerated.
There are rumors he can be a bit of a lush and/or lecher when he's not working. These rumors can be neither confirmed nor denied.


History:
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Masamune was a founding member of Soul Society. Some legends say he and Izanagi-no-Mikoto were the very first true Shinigami, rather than the more roughly-arraigned "gods" that the spiritually-powerful styled themselves as in the days long gone. Masamune was the one who discovered how to forge Zanpakuto that struck true and purified the spirits of Hollows, and adapted themselves to the Shinigami.
Masamune has tried, many times, to train an apprentice to his own level. None met his expectations. Some of those souls re-entered the cycle and ended up as famed Mortal swordsmiths in days past. His most recent apprentices were Sanbo Kojin and Shen Yi. Kojin lacked the necessary spiritual power to truly work the Forge, and her pride was getting in the way of learning her lessons. But meanwhile, Shen Yi was both too prideful, and too treacherous at times. His "rivalry" with Kojin was kept from escalating only by Masamune's firm hand. But even with that, his skill and dedication were greater, despite often lacking the rigorous standards necessary for properly forging Asauchi. When he left, the Black Tortoise King was again left alone again at the Forge, to continue his long-standing Duty. Part of him didn't mind, even though a part of him felt so tired...


Abilities:
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General Abilities:
Spoiler: General
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General Physical Ability: Masamune is a strong man with great physical endurance, no matter what form his body happens to be in. He experiences a minor increase in general ability when he's in his "younger" forms, but it's not a huge jump by any means. His general speed and reflexes are quite good, though certainly not to the scale of some of his peers.
Intelligence: Masamune is a weaponsmith. While his primary duties revolve around Asauchi, which all take a similar form, he is capable of making basically any weapon carried and used by one or two people, even modern weapons (he reads!). He has understandings of things like vehicle design and the like, but not the same skill and knowledge, not the same instinctive grasp of "how does it work". He's cunning and wise, though his knowledge levels in things not related to weapons, metallurgy, materials science, and connected subjects is much closer to the average level.
Weaponsmith: Masamune could probably make a hyper-lethal weapon from a stick, a pack of gum, and some duct tape. When in his Forge, he can make basically anything he thinks of. And with that knowledge of how to create, he has an understanding of how to break down, given the chance.
Languages: Masamune speaks man, many languages. More than a couple of which are dead, or nearly so.


Shinigami Abilities:
Spoiler: Soul Reaper
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Royal Guard Contribution - Zanpakuto/Asauchi: Soul-cutting swords are what, quite literally, set Shinigami apart from other empowered souls in existence. They provide an easy way for Hollows to be purified, rather than just destroyed or dissipated. It allows Shikai and Bankai. Some say that the forged connection between Shinigami and Blade, even the weak ones in Shinigami who have not attained even Shikai yet, is what makes all their fighting skills purifying weaponry, rather than just select Kido. As few have ever tried to test this theory, it is difficult to know for sure.
Whispered rumors and legends say that the souls of Hollows, Menos, or even Shinigami are used to make the Shallow Hilts.
The truth is...more complicated.
One of the sayings that Masamune takes very literally is "the life is in the Blood". He discovered, all those years ago, that if the metals used in making an Ausachi were treated with special oils that have the blood of a Shinigami mixed in, they naturally mold themselves to their wielder. If some of the metal is instead treated with oils containing the blood of a Hollow, purification comes naturally. If quenched in an oil that contains both, the sword's essence is perfectly balanced.
So the truth is that, over the centuries, a combination of "executed" Shinigami prisoners (they did end up dead...) and regular donations from the Royal Guard (including himself) have allowed Masamune to build a stockpile of this essential oil concoction. All blood used in this manner is volunteered, even from the prisoners; attempting to use blood from a Shinigami that was forcibly taken produces a warped blade in both shape and soul.
As for the Hollow blood, when the Royal Guard would hunt Menos, all those years ago, many were brought to special holding cells and farmed for their blood, again producing a stockpile.
When Masamune crafts an Ausachi, before he goes into the Forge of Souls, he gathers samples of metals treated with Shinigami blood, those treated with Hollow blood, those treated with special Kido, and "plain" samples of metal. These are all masterfully layered and forged together using the special blue flames within the heart of the Forge into a single weapon, whose nature is ultimately Shingami; the Hollow blood is used to help "align" the weapon just enough to make purification easy, and (according to Masamune) to give the Shikai and Bankai of Shinigami the ability to be much more esoteric, if possible.
Because he has crafted every Asauchi in existence, Masamune has an awareness of the general location of any and all Shinigami Zanpakuto that are within 100 miles of his person. He has at least some innate understanding of what a Shinigami blade is capable of, if he sees it and can study it for at least a few moments; he cannot understand powers at a release level higher than the sword is currently at (he cannot know a Bankai's powers if he sees the Shikai), and just because he knows what they can do doesn't mean he understands the best counter to them.

Five Elements Theory - Water: "Now you put water in a cup, it becomes the cup; You put water into a bottle it becomes the bottle; You put it in a teapot it becomes the teapot. Now water can flow or it can crash. Be water, my friend."
Masamune often asserts that life is in the blood, and life needs Water. Water is one of the most eternal forces; what fire cannot burn away, in time, water can wear down. Water changes to fit the situation, but its essence stays the same. Water exists in cycles; what has happened before may well happen again.
Masamune reflects this in his work, in that all Asauchi change and adapt to their wielder, and ultimately have 3 forms. As well, Masamune himself has 3 physical forms he takes, depending on the situation.

Oken: Masamune possess the Oken. Primarily speaking, this means he is able to pass into the Royal Realm without special protection. Due to being bonded to his body, particularly his bones, he has slightly increased durability, though primarily against attacks that might break his bones.

Hakuda Expert: Masamune is quite competent in fighting with no weapon, though it is not his preferred method. He tends to favor smooth, flowing styles of combat that emphasize redirection of force, though he has a fair bit of knowledge of the recent form known as "Jeet Kun Do".
Zanjutsu Master: Masamune is an artist when fighting with a sword. His Zanpakuto is practically an extension of his body. While not the finest swordsman in existence, he is certainly in one of the top groupings of such. His style tends to favor flowing movements, quick but powerful cuts, and redirecting an enemy's blade(s).
Hoho Expert: Masamune is quite competent in moving quickly, and generally able to make some basic maneuvers and changes during said high-speed movement. However, he does not know any of the more esoteric Shunpo techniques, and he's certainly not going to win foot races.
Kido Master: Masamune is very skilled and knowledgeable in Kido, especially those relating to his tradecraft. He tends to prefer Hado that conjure water, ice, or lightning, and Bakudo that conjure barriers. Still, he knows both up into the 80s, and is able to chantlessly cast up to the 40s with ease. The one higher-level Kido he can use chantless is Hado 88, which he has a peculiar affinity for. He can only achieve about 50% power without the chant, but it's still an impressive feat. He really, really likes that Hado.


Zanpakuto: Seitou
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Description: When at "rest", Seitou takes the form of a walking stick, about 3.5 feet long, with a knobby, gnarled head. In this form, it's still quite hard, and Masamune sometimes uses it as a weapon. If he wishes to reveal its true sealed form, Masamune merely wills it, and the wood seems to dissolve around his sword, which appears sheathed in his hand. It is a katana of excellent make, with the handle and scabbard being a deep blue. Faint wave patterns decorate the handle, guard, and scabbard, and the blade is obviously of phenomenal quality and beauty.
Spirit: Seitou takes the form of an ancient warlord, one clad in sea-blue armor that's hard to pin down to a single era, a sword whose shape always seems to change belted at his waist, and a simple robe over his shoulders. He is terse, but friendly, simply choosing to use the minimum number of words to maximum effect.
Inner World: A grassy plateau, with one side being a smooth cliff overlooking a shining bay connected to an ocean. There is a small stairway down the cliff to a small beach with white sands. A bit of distance away from the cliff on the plateau is a collection of simple, sturdy buildings that house a few weapon forges and workshops, as well as storage for quite a few weapons.

Shikai:
Spoiler: The Life Sword
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Release Command: Scald, Seitou!
Description: Seitou is surrounded in a cloud of hissing steam, before being revealed as a gigantic blade, one with a long hilt, and a single, wickedly-sharp edge on it. It's not quite a slab of metal but it's not far off, and the spine of the weapon has a handle integrated for situations that need shorter reach.
Sometimes, though, when the steam clears, instead there's just a slightly longer katana, with a gleaming edge and a deep blue coloration, flecked with silver and white.
Power(s):Seitou's released form is large, but (at least for Masamune) just as light and nimble as its sealed form. It's a heavy blade when striking, though, essentially "cheating" a bit when it comes to ease of use versus a "normal" blade that size.
-Be Like Water: Seitou can effortlessly change shape between its larger and smaller form, a flash of steam surrounding it each time. Or, if one is observant enough, they'll realize the blade is actually turning into steam. In point of fact, Masamune can do this without even changing shape, making his blade (except for the "core" of the hilt) intangible at will, though in that form it's not much direct use beyond not taking much in the way of damage. This realization leads to the realization that the solid blades are actually ice, in a very dense, very strong crystalline matrix pattern that makes them as strong or stronger than steel.
-Water Can Burn: When in its smaller form, Seitou can emit small, dense clouds of steam. These clouds can be directed by his will, though he can produce at most 2 clouds at a time, each about the size of his torso. The clouds can move quite rapidly, and independent of each other.
-Blade of Blades: When in its smaller form, Seitou is extraordinarily hard and sharp. Masamune is able to perform slashes so fast and powerful they briefly create vacuums, such that some opponents may be drawn toward him, or at least put off-balance for critical moments.


Bankai:
Spoiler: The Steam Fortress
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Suijoukaku
Description: In a roiling cloud of steam, Masamune emerges wearing a suit of armor. Styled after O-Yoroi, but also European Plate, it has smooth, graceful plates of armor supplemented by flexible padding and strips of armor. His face is exposed, allowing his now-black beard to run free, and his long hair comes out underneath his helmet. The armor as a whole seems to be slightly "sweating" water, especially at the joints; if Masamune stands in one place long enough, it may start to pool and puddle at his feet. On his back is a somewhat rectangular construct with three pipes on each side, slowly but steadily emitting steam. In his hands, he bears Seitou's Shikai form
Power(s): Suijoukaku bears all the powers of Seitou, in addition to its new abilities.
-Mountainous Ice: The armor Masamune wears is incredibly resilient to damage, despite actually being plates of ice. At will, he can draw in and harden the water his armor is "sweating", doubling his protection but halving his speed and agility. It takes mere moments for him to do this in either direction.
-Armory of Blades: The blade of Suijoukaku can now change into virtually any form Masamune desires or imagines, all in a flash of steam. All such weapons (which are restricted to any hand-held non-mechanized melee weapons) are as hard and strong and sharp as Seitou's smaller form, and can thus perform the "vacuum slash" move. This shifting of forms can even give Masamune two blades at once. After all, if water can take one shape, it can take many shapes; in the hands of a master smith and swordsman, is not any blade a Blade of Blades?
-Cycle of Water: Masamune can now summon up blasts of liquid water or barrages of spikes and chunks of ice as well as scalding steam, and maintain up to 5 masses equal to his torso in size.
-Inevitable Erosion: Masamune can dissolve everything but the pack on his back (which is made of actual metal, not water) and congeal it all into a huge mass of water, which he can cycle through any of the 3 states of matter. He has complete control over this mass, and can mix it with the masses he can summon beyond his weapons. In this form he can make one large ice construct, deluge of water, or dense cloud of incredibly hot steam.


The Realm Of The Tortoise King
Spoiler: Forge of the Gods
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Masamune's section of the Royal Realm is in two parts, the overall Smith's Island, and then the Forge of the Gods itself.
-Smith's Island: Masamune's residence takes the appearance of an island surrounded by a ring of ocean (that never seems to actually spill out beyond itself), with white-sanded beaches gently lapped by waves from the false ocean. On the "south" beach, there are several cottages just past the sand, one of them being Masamune's own home, the others being guest homes, plus a few buildings holding miscellaneous supplies and the like.
Masaumen's cottage is a fairly modern, Western design, with a mix of decorations and amenities from many time periods and regions inside. There's a fairly extensive library (that's bigger than it ought to be considering the size of the home).
As one travels further into the island, the terrain becomes rockier, until at the center, there's a large hill/very small mountain, with a flat top dusted with snow. In the center of this plateau rests the Forge of the Gods.
-Forge Of The Gods: Here is Masamune's crowning achievement. This forge holds within its walls much more space than its exterior would suggest. Entering, one walks down a hall with examples of weapons and armor from all of history, and all over the world, complete with plaques giving names and history.
Next up are several storage rooms, holding stacks of bars and ingots of metal; at a glance it's all steel, but a closer look reveals varying types, some of which have...odd...shines to them. Beyond these rooms are more storage, now holding various materials (leather, wood, cloth, etc.) to finish sword construction.
After that is the Sacred Oil storage, where oils for treating the metals, as well as for quenching, reside.
The Workshop is at the center of it all. It is one of the least modern parts of the area, though there are touches here and there. Certainly some of the powered tools look to help with things like polishing, or drawing out raw pieces of metal. But most of the work is done with several furnaces that surround a pit of bright blue flames, which feeds each of them; Masamune seems to be able to control the rate, allowing him to control the temperature of each of the furnaces for the job needed. Arrayed just in front of the furnaces, whose surfaces glow with ancient runes and symbols, are an array of anvils and hammers. Masamune typically moves between various anvils at various times. He has different-sized hammers depending on what he's working on, though there's one particularly large and heavy one he favors using much of the time. Quenching tanks rest nearby, filled with strange oil with an odd smell.
This is where Masamune can often be found, creating Ausachi to be delivered to Soul Society, so as to properly equip each generation of Shinigami. It is his most sacred, highest duty, at least in his own eyes. Perhaps if he ever gets a worthwhile apprentice, then he can rest...