Tweaked my stats some more towards being a twinky assassin. Still assuming no move changes for now.
E-mouse shows up when the tournament falls apart and things start getting serious/political: He appears as a neutral unit on the next map and (if possible) sneaks through an unguarded segment of the map to loot items and documents as evidence of what's going on. He can be recruited by having Ruro talk to him. (And/or one of the others from Ruro's hometown talk with him; not sure if this is easily coded?) Resulting dialog:
E-mouse: "Who are you?"
Ruro: "Mousey? Is that you?"
E-mouse: "... what? How'd you know my name?"
Ruro: "It's me, Ruro! We were spreading JUSTICE!! at the tournament, but then this happened, and we're trying to figure it out..."
E-mouse: "... I'd like to trust you, but I'm not sure you're actually--"
Ruro: "Oh, stop being paranoid, Mouse. I liked it better when you were drunk in the fields that one time and asked to squeeze my b--"
E-mouse: "OKAY, okay, I'll trust you. I think I'll tag along."
Ruro: "Great!"
Tweak dialog contents as needed if others can recruit, but I'd like to keep the joke
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Late-game, I'd also like it if he/I could also be a prerequisite for getting one or two of the Assassin advancement items, after a boss fight with a major leader of (one of the warring nations), where they make it very clear they aren't going to listen to reason. (Optional extra prerequisites to reflect this: Have E-mouse 1. be involved in the battle and possibly 2. Have to talk to the boss at least once or twice [and fail at diplomatic resolution] to be fully certain.) After that battle, if the necessary prerequisites are met, E-mouse says "... now I'm mad." and says he needs to get something from town. He later returns with one or two Assassin promotion items, saying that he'd stashed them away in town in case he had to start being ruthless, and that the fight convinced him that there's nothing left to hold back. But he's more willing to trust the Lords with deciding on how to use them than himself.