She's gross and is ultimately at the end of the day a predator. 21 year old girls should NOT be going after 13 year old boys period. 21 year old men should NOT be going after 13 year old girls. Even if those children are physically developed doesn't mean shit, they are still 13 on the inside. 13 year olds are easy to manipulate and taken advantage of and are still children. How good looking she may be is completely irrelevant to this case, if a good looking 21 year old man went after a 13 year old it would still be fucking gross. This girl CLEARLY has issues,  and probably an ulterior motive, to be preying on that young boy and thoroughly deserves the prison sentence she and all other sexual predators like her are  likely to receive.
I'm sure all of you remember how fucking stupid you were when you were 13. Just because the guy/girl is pretty doesn't mean they aren't sickos.