Amphitrite is the a series who lives in the sea with her father Nereus and 49 other sisters including Thetis, who is also depicted on the cover of her book "Amphitrite the Bubbly". She participates in the Temple Games and pretends to go as Thetis since Thetis prefers to stay in the sea and Thetis was the on invited. She develops a crush on Poseidon and helps him be a better competitor.
Amphitrite is very competitive in the myth, so it will be likely she is competitive in the book.
Suzanne Williams confirmed the mystery girl on the cover of Amphitrite the Bubbly is Thetis