|“||You will be fine, Frank Zhang. Keep practicing those sea life transformations. The koi fish is good, but try for a Portuguese man-of-war. Remember what I showed you. It’s all in the breathing.||”|
–Bythos to Frank Zhang, in The Mark of Athena
Bythos and his comrades rescue Leo Valdez, Hazel Levesque, and Frank Zhang from drowning in the ocean after they defeated the Skolopendra. He later interrogates Frank in order to figure out why they were here and whether they could trust them. He later shows up with Frank and both he and Aphros discuss the situation with them. He later agrees to send a group of mer-heroes on a quest to save the creatures trapped by Phorcys and Keto.
Kymopoleia reveals to Jason and Percy that Hazel had made such an impression on the merpeople and Aphros and Bythos that they have gone to war trying to keep Gaea's allies away from Long Island. However, Kymopoleia states that it remains to be seen whether or not they will survive. Percy remembers the two, commenting that they didn't want to meet him.
He has green skin and a scraggly brown beard. His long hair is always tied back in a seaweed bandana. A pair of lobster claws stick up from his head like horns, turning and snapping at random. He has the forelegs of a blue-green horse, like a centaur, but toward the back, his horse body morphs into a long fishy tail about ten feet long, with a rainbow-colored, V-shaped tail fin.