I actually had to boot up my gamecube and play some Paper Mario TTYD to get some ideas for MU!
She's 100% a Thief!(or whatever those dudes are called lol)
Gameplay wise she'd be a temp partner kind of like Flavio but she actually actively helps out and you could control her for the majority of the fights until the end of Chapter 1.
After Chapter 1 you'll have the option to find her hiding spot on future Chapters and have an optional boss/mini boss fight.
Every time you fight her she'll have new tricks up her sleeves!
(This includes DLC Chapters too)
(I'm saying this like I'm making a game for this LOL! Heck I wish!)

