I dunno if this has been pointed out, but I just found this in an article about the Wii version:
Co-Optimus: How extensive is the character customizability? Will I be able to bring this character into the co-op session?
Dustin: For the Wii, players are given two characters to choose from, male and female versions of a newly-hired experimental equipment technician. Either character will be playable in the single player campaign, and in co-op the second player will get to use the other character. There is a fun customization option unlocked for players who complete the game on the hardest difficulty. It’s one of several unlockables the player can earn by accomplishing various feats throughout the game.