Gemini Kanon was the younger twin brother of Gemini Saga, and consequently having the same sign, which made Kanon was chosen to replace his brother as Gemini saint; Kanon, unlike his brother, was ambitious for power and wanted to rule the earth, and tried to convince his brother to try to steal the title of Pope (which was given to Aiolos), and with his power, dethroning Athena. This made Kanon was the famous villain style, known as the "evil twin".
Saga, imprisoned his brother in the prison of Cape Sounion, and just go out there, be a god allowed. But it was too late, and the face of Saga's disease, woke up and followed his plan to take the post of pope for you.
Kanon, was saved by Athena (still a baby) several times of death, until he found the ruins of the Temple of Poseidon, and the release of the amphora where he was also arrested; Poseidon thinks that Kanon was one of his servants and Kanon only confirms your guesses (proclaiming himself, Sea Dragon), and so God thinks Gemini woke it was time to fight Athena, and trust him, wake him from his sleep within your host, Julian Solo.
Kanon violates the promise, and like his brother, falsely is a god and takes on the power of his army, deceiving them.
After the death of Saga, Kanon send declare war on Sekai, hoping Athena and Poseidon were defeated, to become the be strongest on earth. The Saints can unwittingly awaken Poseidon, ruining 13 years Gemini planning, which is overlooked by Ikki and Sorento; after discovering that Athena saved him from death, he returns to his servant, taking his brother's posture and dying in the war against Hades.