We've run this game with Ameiko as a PC in a 2 man party, and it worked just fine. There's a few changes that need to be made:
- Obviously, you need to rebalance the entire module to work as a solo 1 character game. This is a significant amount of work. But you'd need to do this whether or not Ameiko was the PC or merely an NPC escortee.
- Ameiko will move from being a Plot Device to the protagonist of the story. Personally, I think this a good thing, as it moves her from being a burden you need to escort endlessly to a fighting princess in charge of her own destiny.
- Minor elements of the story revolving around leaving Ameiko in safety while the PC does Dangerous Thing. Some minor bending of logic works when Ameiko becomes the highest level PC instead, thus the most logical choice to do Dangerous Thing.
- The plot centers around Ameiko as the most important and special character. In a group game with one player being Ameiko, this can be a problem. In a solo game, it's actively an advantage.
- Ameiko needs rebuilding (you don't want a main PC with aristocrat levels). However, nothing about the game assumes she will have access to certain abilities, so it's up to the player what she wants Ameiko to be. We went with Paladin/Bard, but there's no reason she couldn't be a cleric or a wizard or a spellthief instead.
The only plot element that needs changing is
Spoiler: Extremely minor, non-specific spoiler
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At some point Ameiko becomes incapacitated by visions she can't adequately communicate, justifying why the PC needs to do A Thing instead of Ameiko. It would be the work of mere minutes to make the visions vaguer, then give them to PC Ameiko directly to drive the adventure.
But Plot-wise, Jade Regent is a fantastic choice for a solo game. Ameiko gets a string of weighty decisions to make, romance options, a cast of interesting NPCs to travel with, and an epic journey to confront her destiny. Kingmaker is the only other AP that comes close to it for automatically-plotted-for-a-solo-PC game.