From The TimeSplitters Wiki

Nikki is a character in TimeSplitters 2 that can be unlocked for arcade. She has a human like face on the front of her head, and a monster like one on the back, same as the one found on the front face of her sister, Jinki. Also, she is a freak in the circus. To unlock, you need a gold medal on Nikki Jinki Bricky challenge in arcade mode.