Goombella, your first partner in the game, is a sassy, intelligent Goomba. She's a graduate of the University of Goom, and demonstrates her education well through her descriptions of locations, citizens, and enemies' weaknesses. She's not one to stay silent when uncomfortable advances are made on her, and will call them out on being a creep without hesitation.
For her voice, I imagine her just sounding like your standard peppy, girly character, maybe even leaning SLIGHTLY into a valley girl accent. Not too hard though, she should still give off a vibe of intelligence.
This role will likely be one of the most line-intensive roles, as I'd like to try to include all of the Tattles for each enemy in the game. Other than that, this character will likely have a large amount of lines throughout the prologue and a good chunk of chapter 1.