Well, if you modify it enough that it's mechanically distinct, then sure. That's what I mean by different niches.
Let me start off by saying that my impressions of these spirits are positive. I harp on various issues because it's easier to point out the parts that don't work than it is the points that do - the majority is solid. Sorry if I seem negative.
Robin appears to have no regular spell list, so a player learns no spells when contracting him. The only spell granted by this caster-class spirit appears to be, "You get one spell when you include/install." A bit odd for a caster to grant fewer spells than other spirits. Besides that, the install grants a unique combination of abilities (int and str), but nothing beyond that, and the only use of STR during this install is to buff damage spells. To that end, Tamamo is a clear choice over him.
If rally spectrum costs 1 round to turn on and stays for free, it might be a good saving grace - it's significantly more powerful than Anderson's passive. I think your "and maintaining" means it doesn't, so I'd call this a hard pick besides the specific spells it grants (below).
Of his spells, a few - Magic Missile, Orb of Electricity, and Chain Lightning - are in use by other spirits. The only totally unbalancing spell on the list is darkbolt, which is scaled as a ~4th-5th level spell but given to the vile darkness domain at 2nd. It's one of the best spells for chameleon because it's so incredibly powerful for a 2nd level spell. Mystic Lash is also quite nice. They're even more powerful combined, which this guy can't do, since he can only use 1 per include.
You say "+2 to int and str." I'd cut out the +2 unless you really like it not scaling. That goes for your saber build as well.
Overall the mechanics of his include and install are solid, but the spirit feels to me like it doesn't do enough.
Odd mechanic with the sword, but I suppose it's unique. The WSAs are all dupes, but I'm having trouble finding unique ones myself. +Int to attacks is quite nice.
A saber with no attack boost, huh? Looks like he has mostly the same powers as the other build, including caster boosts with no casting restriction. I'm not sure I like that, because it gives a number of options I tried to avoid. I like the stock soul having only 1 caster install/day, so they can't use one to cast full-CL buffs in the morning unless they're blowing their only one on it (or spent feats or ACFs to do so). With this they have that option.
The spells are dupes with your previous spirit but not with any of mine.
I'd say this build looks more like a caster than a saber to me, at least compared to the ones I've made. No attack boost, int boost, no spell restriction, plus mostly-passive aura.
As a style note, my iconic servant soul is female, as are all pronouns referring to her. You seem to be referring to the spirit itself in the abilities, saying "The spirit can do X, and gets Y, and he gets Z." My format was closer to, "The Spirit can do X. The servant soul therefore gets Y, and she gets Z."
Overall, this second Robin feels like a crib of the first. It does roughly the same things right but does a few extra things wrong.
Daggers are a bit boring, and a number of spirits grant them in some form - in practice, I think Tawrich and Zarich are closer to Sais. Your WSA progression is a bit off of mine. I granted +1s at 7,10,13,and 16. And any time I granted a +2, I'd give it 1 point above the appropriate +1 (hence the common 7/11/16 or Elizabeth's 8/13/16). If I granted any +3s I'd give it at the midpoint (hence Atalanta's Splitting at 13). I don't think I granted any WSAs at 12th, since the progression was designed to never grant a WSA and a + enhancement at the same level, but let me know if I did. I think one reason it looks odd because you grant a total of +5 to weapon abilities. I kept all of my weapons maxed at +9 at 16th level. If a character builds Spiritual Vassal (Angra), they'd get a +11-equivalent weapon. (they get broken half the time he fights! don't sound like epic weapons to me.)
I'm not sure what "counts as a hexblade" means. I assume you had some specific use in mind for it, but they can't use hexblade wants or anything since they lack those spells on their spell list. Is that what you meant? What is meant by "weapon useage"? Does one of your dragon mag WSAs require hexblade?
His first include phantasm is drastically underpowered. Something that happens so rarely should either impart massive advantage, like spell turning, or at least cost fewer than 3 rounds. I mean, it would be worth using if you could just throw on your angra to ditch a curse, but it look like it only works if it's actively being cast.
Installs can't grant bonus to will saves, since the bonus is the fixing up difference between a bad and good progression. In other words, a bonus to will saves = +0 will saves, at least RAI when I wrote the save bonus rules. That's why no casters grant save bonuses.
Verg Avesta is a decent anti-boss power but it's one I'd generally try not to have to use as a player, since 1:2 (or even 1:1) HP trades are always bad for the player. If you're fighting mooks, then they have a combined total HP way higher than you. If fighting a boss, then YOU BET the boss has more HP than you. Vile Damage is mean to PCs but almost never matters to an NPC.
Basically, this is a PC-killing NPC, but a PC would find the powers nearly useless. It's interesting, unique, and full of flavorful. Just not... useful for a PC in combat, is all.
The Servant Soul spellcasing already includes the line, "Servant souls incur no penalty for casting spells while holding a weapon granted by any servant soul class feature" so I have you covered there. Flaming Burst can't be gained without flaming, and both are granted by Gawain. Returning and Brilliant Energy are also granted by existing spirits. By giving Brilliant Energy at 16th level, the user now holds a +12 weapon. With two levels in Spiritual Vassal it's +13. Yikes. Note that Brilliant Energy means that the servant is 100% unable to damage or even hit constructs or undead.
I avoided using variable-round-consuming phantasms as the first phantasm, since they are used in Includes, which don't spend rounds but rather terminate upon conclusion. As-is there's no penalty for someone including Phyriss and "spending" 6 rounds, because why spend only 2 or 4? Maybe put a clause in explaining what restrictions, if any, are placed on include-only use.
His base install powers are pretty weak. All casters gave some extra boost to balance lack of attack and save bonus, although for some (like Anderson) it was pretty token, since the spirit is carried by other class abilities. Still, besides the int bonus and (standard) CL boost, his install seems like a direct nerf: loss of spell access and all damage swapped to the most common resistance/immunity.
Blast Wave is an odd phantasm, in that I never build one like it, so I have little for direct comparison. It nukes the install for a crapton of damage then exits. "Vulnerable to fire" means instead of 4.5/Round it deals about 6.75 and, as written, requires no attack roll and grants no save. Basically, Phyriss has a 1/Day nuke, and the rest of the spirit's powers are weak enough that the nuke is the only defining feature. He makes a good backup spirit for someone that has most mechanical niches already covered, and just wants the occasional explosion. That, or Devoted Soul (Phyriss) and end every encounter in one shot. Because if you went Spirit Affinity (Phyriss) and Spirit Devotion (Phyriss), with 18 starting con and a +2 item by level 6, you're now 3/day shooting 100+ damage to anyone not immune to fire 3/day. Basically, you end every encounter in 1 shot, or are basically useless. And that says nothing about starting as a dragonborn mongrelfolk...
Odd that the include phantasm is variable-round and the install phantasm has no round deduction mechanic. I'd rebalance the blast wave, then swap them. Especially since empowering/maximizing spells during an include is lackluster, given the halved caster level outside installs.
The mechanics are unique, and they give me some ideas. But the implementation of the phantasms leaves a few rules holes, and supports a degenerate playstyle. The spell list is solid.
That was a lot of typing. Fizban, I'll respond to you later sometime. Tomorrow maybe.