Sarah seems to be mature for her age, like Ellie, as she is calm and collected when she claims she's 'fine' to Tommy, mere minutes after Joel shot their infected neighbor. She obviously cares about him, as she somehow manages to scrounge up enough money to buy him a (presumably) quite expensive birthday present. Sarah seems to have a similar sense of humor to Joel, as she trades jabs with her father during the prologue without missing a beat.
