Just repeating what others said, but she's just a character that only appears in the artwork to give Liberty City an atmosphere before anyone plays it.
Martini Girl in VC, gives the sense of drinks, clubs, pools, hot weather, and babes before you even buy the game.
Rochelle (?) in SA, gives the sense of the celebrity/movie stars craze kicking off.
Lola in IV gives the sense of prostitution and corruption in the city.
My thoughts, anyhow.