Crystal is a character mentioned in Uncharted 4: A Thief's End.
Not much is known about Crystal's early life. She was the girlfriend of Sam Drake during his teenage years, who had an on-and-off relationship.
Events of Uncharted 4: A Thief's End
While Nathan and Sam were sneaking out of the Saint Francis' Boy's Home, Nate asked Sam if he was "still dating that girl." Sam replied by saying "Who... Crystal? Eh... on again, off again. Currently... off again," implying that the two had broken up once again at that time.