|“||I think he means gemini like doubled - half man, half snake. That's what his people are called. He's a geminus, singular.||”|
Gemini are a species of half-human, half-snakes who were the original Athenians during Kekrops' reign as king. They are the self-proclaimed first and last children of Gaea, and served her during the Second Giant War.
Similar to scythian dracaenae, gemini are a serpentine race of monsters, but instead of having two serpent trunks for legs, gemini only have one. Additionally, their upper halves are described to be more human looking than those of dracaenae.
The city that would become Athens was founded by the earthborn King Kekrops. During the dispute between Poseidon and Athena, Kekrops chose the latter as Athens' patron god. He was the first to offer sacrifices to Athena, and built her first shrine on the Acropolis.
Upon the birth of Erikthonius, the mortal son of Athena and Hephaestus, Kekrops' daughters were tasked with looking after the boy while he was placed in a wooden box with a serpent to become immortal. Overcome by curiosity, the princesses opened the box and were driven mad as a result, flinging themselves off the Acropolis in a fit of insanity. As punishment for Kekrops, he was usurped by Erikthonius as king of Athens and was driven underground with the rest of the gemini by the human inhabitants.
When the Argo II arrives in Piraeus on the outskirts of Athens, the ship is approached by three gemini, one of which introduces himself as King Kekrops. He offers the Seven safe passage to the Acropolis, which is heavily guarded by the Giant Army aboveground. However, so as to avoid detection, Kekrops says he will only lead a maximum of three demigods through the gemini's underground passages.
Suspecting treachery on Kekrops' part, Piper McLean remembers the "song of the snakes" from a Cherokee myth. Singing "Summertime", her father's favorite song, Piper coerces Kekrops into admitting that he and the gemini are serving Gaea in return for place in the upper world.
Together with Percy Jackson and Annabeth Chase, Piper musically controls Kekrops and the gemini to lead them through the underground passages to the Acropolis. Upon reaching their destination, Piper commands the gemini to scout the area for signs of the Giant Army. Kekrops returns alone, signalling for the demigods to hurry as the ceremony to awaken Gaea is nearly complete.
During the battle in the Acropolis, Kekrops and the gemini join the fight against the Seven. Hazel causes the ground beneath the gemini to collapse, reducing their army to only about six. As Kekrops orders them to flee, Frank eliminates the remaining gemini with his arrows.
- The songs Piper used to control the gemini were "Summertime" and "Happy Trails".
- There are snake people similar to gemini in Cherokee myths. One of the myths is about a warrior who learns the song of the snake people, but had to sacrifice his wife for the power. Piper drew inspiration from this myth when she sang to control the gemini.
- An alternative name is diphuês, which comes from the ancient Greek dis and phuê, and means "double in nature" or "binary".