From The TimeSplitters Wiki

Jo-Beth Casey
Unlocked by	TS2: Elite League > One Shot Thrills > Babes In The Woods with Silver or Better TS3:Default
Species	Human
Size	Normal
Accuracy	5
Agility	5
Stamina	5
Fire Proof	5
Shock Proof	5
Default AI	3
Native TimeZone	1994
First Appearance	TS2 Arcade League
Relations	Mary-Beth Casey
Games	TS2, TSFP

TS1 Gesture	N/A	TSFP Gesture	Begins dancing with hands out in front, then up in the air says "Shake your money maker!"
TS2 Gallery	Mary-Beth Casey's younger sister. Jo-Beth likes to sneak out of her parents' house at weekends to fight crime and kill monsters. Her high school show and tell sessions are really something quite special.	TSFP Gallery	F'Sure

In TimeSplitters 2, Jo-Beth was a rather nerdy girl with glasses. However, in her return in TimeSplitters: Future Perfect her attire has become rather risque, with a skirt that's almost falling off and a shirt with "SLUT" written across her now much larger breasts. In TSFP she helps Cortez out in The Mansion of Madness, and develops something of a crush on him, checking several times that he is looking at her big butt.

Trivia

Jo-Beth Casey has same looking face than Tomb Raider's main character, Lara Croft. That tells that somehow, Eidos, the maker of Tomb Raider series, has used some artwork for Jo-Beth. But this is only guess.