One character I'd buy in a heartbeat is "The Cougar". She's currently featured as the mascot of Morningwood High School -- see the center of the gym. When you launch her she'd make this sound and would fit in well with the "sexual innuendo" theme of the game.
What do other Pain fans think?
LOL. would love that character. Great choice in sound effects also, very fitting. They should add a few additional trophies as well. for instance, hit 3 male characters in one run, or knock out 100 female characters over a lifetime use of "The Cougar".