From The TimeSplitters Wiki

Nikki
Unlocked by	Elite League > One Shot Thrills > Nikki Jinki Bricky with Gold or Better
Species	Human?
Size	Normal
Accuracy	6
Agility	3
Stamina	6
Fire Proof	N/A
Shock Proof	N/A
Default AI	3
Native TimeZone	Unknown
First Appearance	TS2
Relations	Jinki (Sister)
Games	TS2

TS1 Gesture	N/A	TSFP Gesture	N/A
TS2 Gallery	Twins from a tiny Oriental village, Nikki and Jinki were sold to the Ringmistress when they were both babes in arms. Strangely, each claims to be the elder sister.	TSFP Gallery	N/A

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.