There's no denying that Once has a complicated family tree, but have you read a certain new page? Poseidon is joining the cast for the second half of season four. It's never been said outright that Ariel's grandfather is Poseidon, but it's sort of implied since her father is Triton, the son of Poseidon in Greek mythology. That's what I expected reading the Poseidon page, but instead his family tree shows him being the father of Ursula. Is Ursula Ariel's aunt or did they do away with the unspoken understanding that Poseidon is Ariel's grandfather?