|“||The main courtyard was filled with warriors - mermen with fish tails from the waist down and human bodies from the waist up, except their skin was blue, which I'd never known before. Some were tending the wounded. Some were sharpening spears or swords. One passed us, swimming in a hurry. His eyes were bright green, like that stuff they put on glow-sticks, and his teeth were shark teeth. They don't show you stuff like that in The Little Mermaid.||”|
Percy Jackson wakes up in Poseidon's palace underneath the sea. There he sees many merpeople wearing battle armor ready to fight the forces of Oceanus and some making underwater graves for fallen merpeople. During the battle, a swarm of mermen attack a large squid, but are losing the fight until Poseidon intervenes and destroys it himself. Oceanus himself looks very much like a merman when Percy sees him on Poseidon's map of the battlefield, except he is much larger and has horns.
Dozens of mermen and mermaids are seen at the underwater merpeople camp that Leo Valdez names Camp Fish-Blood. They are seen swimming through the streets, in courtyards and practicing with tridents in the area. They are trained by two ichthyocentaurs named Aphros and Bythos. Much like Camp Half-Blood, merpeople go to this camp in order to train to become effective warriors, similar to demigods. Triton himself even trained at the camp during his younger days. When Leo Valdez, Frank Zhang, and Hazel Levesque informed Aphros and Bythos about the Georgia Aquarium lead by Keto and Phorcys, they sent a group of merpeople to free them. Bill, one of the most famous and skilled of the merpeople, was chosen for the quest.
Because of their underwater existence and anatomy, merpeople are skilled swimmers and can survive even in the deepest parts of the ocean. Their bodies also allow them to survive the freezing cold temperatures at the bottom of the ocean. Much like demigods, they are trained in combat as a way to survive the dangers of the ocean.
They appear human, but instead of feet they have fish-like tails. They have shark-like teeth, leathery green, blue, coral red to inky black skin color, and bright green or yellow eyes, like glow sticks. Poseidon's son and heir, Triton, appears to be an alternate form of a merman with two tails rather than one. They are sentient beings found in many myths other than the Greek ones.
- Merpeople may have a way to travel across land, as Bill was able to save the monsters in the Georgia Aquarium, despite it being in a land-locked area.
- In Greek mythology, merpeople were called tritons, after their ancestral sea god and described as having claws, gills, shark-like skin, fangs, green seaweed-like hair, and fish tails.
- Glaucus is another merman in Greek mythology. He was once a fisherman who ate some magical herbs that turned his hands into fins, and his legs into a tail. Later he was given the gift of prophecy and was made an oracle of the sea. He's mentioned in Percy Jackson's Greek Heroes when Jason and the Argonauts find Hercules missing.