From The TimeSplitters Wiki

Mary-Beth Casey
Unlocked by	Complete the second set of three Story missions
Species	Human
Size	Normal
Accuracy	N/A
Agility	N/A
Stamina	N/A
Fire Proof	N/A
Shock Proof	N/A
Default AI	N/A
Native TimeZone	1965
First Appearance	TS1 Story Mode
Relations	Jo-Beth Casey (Sister)
Games	TS1

Mary-Beth Casey, much like her younger sister, Jo-Beth Casey travels into a Mansion to fight zombies. Mary-Beth only appears in TimeSplitters 1. She is one of the two playable characters for the Mansion mission, the other being Peekaboo Jones.

TS1 Gesture	Jumps up in the air, claps then stands still.	TSFP Gesture	N/A
TS2 Gallery	N/A	TSFP Gallery	N/A