Ooooo, I like it. So your two know she is to stabilize the crystal, but they don't know how that is supposed to work. Perhaps they thin her just being there will stabilize it. Thats great. 

I think her Atlantean heritage would be the thing that would make her feel different. I can see them maybe aging normally until they reach a certain age for the sake of the rp that way she's not like 5 years old for 100 years haha XD maybe when they reach age 20 or 21 their aging slows dramatically tot he aging rate of those in the actual movie? 

How about my female, in being sent to the surface, gets put into foster care and moved around from place to place. And maybe in being in foster care is where she meets my male. After that they could get pretty close and she could essentially become her foster brother, the two getting moved around together because they refuse to be separated?

And that last idea works as well, but I have to wonder what they pan to do about the crystal when she's gone? After all, that would be the end of their line if she were to die, so what would happen when the crystal needed stabilizing years later? Perhaps they think that will be past their time(when their dead) so they won't have to deal with it? Or maybe something else?