Moss'heart is Sun'spirit's brother. He was born deaf but that doesn't stop him from being the clan menace. As an apprentice it wasn't uncommon to hear him get in trouble at clan borders or get stuck in a tree. He also liked to play pranks on any cat who stayed in camp for long amounts of time.
He is an extremely protective sibling (because his littermate got injured by another cat), as kits, when both he and Sun first arrived in the clan (they were born in a different clan), he didn't let any other kit in the nursery go around Sun. At least that was until Deer and Sparrow hard-headed their way through, becoming a close friend group.