Who's This Random Girl?

"Here comes the new girl." Sirius says looking down at his food, while James moved to leave. "Where are you going? She's not gonna kill you prongs." Remus says looking at his book. James laughs. "You don't know that for all we know, she might try to..." He sits back down despite himself.
"Hello James. Or more widely known as the Potter Heir."
"Fuck off Crystal. You don't know me! You never did, and you never will!" James Potter says as he storms out of the great hall.
"How do you know each other?" Sirius asks confused and flabbergasted.

Who's the girl that comes and wins everyone over, and leaves James Potter in the dust.
Why does only James Potter know who she is?
Who's this new girl?
Why does she look like James?
Why does she know him better than his best mates do?