It's from DOA5 vanilla, you had to beat Tina on stage 8 on legendary without losing I think. They're auto unlocked in LR I think.
Well, that is weird. Well, so, she will be the "boss" until i beat the entire arcade mode without losing?
She's just straight up the final opponent for Legend Arcade; just as Christie will be in her place for Time Attack Legend and Lisa for Survival Legend.
It is if you're like me and have a super religious mother who would bite your head off if she saw that :-/