Gemini Kanon is a recurring character in Saint Seiya . While he does not appear for the first half of the series, he's key character for the story, as he serves as the overarching antagonist of the Sanctuary Arc, one of the two main antagonists (Alongside Poseidon) of the Poseidon Arc and a supporting character in the Hades arc. He is the younger twin brother of Gemini Saga and he was the first one to notice the evil inside of his brother and he was the mastermind behind the awakening of Poseidon.
He was voiced by Ryōtarō Okiayu.
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). Saga refused and stated it was a foolish thing to do. But Kanon held tight in his decision and stated that even Saga had the same will, as he knew of the dark presence that lived inside his heart.
Not wanting to hear him anymore, Saga, imprisoned his brother in the prison of Cape Sounion, as a punishment for having such evil thoughts. Kanon tried to make his brother release him, but it was pointless. The younger twin then stated that one day the evil side of Saga will take control and try to kill Athena. Saga stated that it wouldn't have never happened, but Kanon already saw it.
During his stay in his prison, Kanon risked death many times due to the ocean rising, until it was saved by a cosmo he didn't recognise. One day, he was able to find Poseidon's trident and extracted it. This brought him to the deceased realm of the sea, where the temple of Poseidon was standing. Once inside, Kanon set the spirit of the god free, as he temporarely possessed his Cloth. Kanon told him that he was Sea Dragon, one of his Marine Generals, and advises him that Athena has recently been reborn on the earth. Poseidon then declares that he will possess the body of the newest member of the Solo family, Julian and then disappears in the body of the young child, not before making Kanon promise that he will not awaken him before Julian will be sixteen years old.
Kanon decides to violate the promise. As he wears the Sea Dragon Cloth, he states that he will use Poseidon as a tool to destroy Athena and then he will overthrow him and become the overlord of the world. It is unknown what Kanon had done during the years that preceeded the beginning of the series. But he probably searched for the rest of the Marine Generals and the soldiers of Poseidon. Somehow, Isaac of the Kraken, one of the generals, knew about his plan.
Once the battle between the Saints and Poseidon started, Kanon made his first appearance as Sea Dragon when Seiya and Shun descend to the realm of the sea. He hits them and then challenges them to destroy the seven pillars that hold the realm of the sea before leaving. While the Saints manage to destroy five pillars, Ikki decides to go directly to the temple of Poseidon in order to defeat him, but Sea Dragon blocks him the way and attacks him with the Galaxian Explosion. Since he recognized the attack and felt a similar cosmo on him, Ikki asks him who he was. Sea Dragon decides to answer him as he took off his helmet. At first, Ikki confused for him as Saga, wondering how he could be still alive. This made Kanon laugh and then he answered that Saga was his older twin brother and that he is the younger one. Kanon also stated unlike his brother, who had two personalities, one good and one evil, he had only an evil personality. Kanon then said that he knew of Ikki's ability of returning from the ashes, so he decides to banish him into another dimension by using the technique called Golden Triangle. Once he did so, he decides to return to his own pillar.
Kanon is then seen talking with Sorrento, who asked him about his true identity. But Kanon answered that he shouldn't ask things that don't regard him. He then returns to his pillar. After that, he could feel that the Saints were challenging Julian, and that the spirit of Poseidon was slowly awakening inside his body. Kanon feared that this could ruin his plans, but before he could go stop them, he was interrupted by Ikki, who was able to return from the other dimension. By using his Gen-Ma-Ken technique, Ikki made Kanon tell all the truth before they engaged in battle. The Saints then asked him where was the jar where Poseidon was once sealed. Kanon answered that it was inside the main pillar of Poseidon's temple and that there was nothing they could do to retrieve it. Ikki then said that the one saving him back at the Cape Sounion prison was Athena. Kanon finds it unbelievable and continues the battle, which is then interrupted by Sorrento, who said that now that Kanon's plan was discovered, it was useless to continue fighting and allowed Ikki to destroy the last pillar. After this, Kanon attacks the Saint repeatedly in the middle of the fury, but he is again stopped by Sorrento, that made Ikki reach his friends. The other Marine General then decided to leave Kanon alone, as he said that at this point, it would not even make sense and that he won't be the overlord of nothing. With this, Kanon is left alone bewildered. In the manga, Kanon decides to defend Athena from Poseidon's trident, as he's revealed repented of what he had done and thinks that this is his punishment (In the anime, it's Seiya who defends Athena from the attack). Kanon then manages to survive and got out of the Realm of the sea alive.
Kanon returned in the Hades Arc, where he asked for apology from Athena. While the goddess seemed to forgive him, the Gold Saint Milo Scorpio didn't want to accept it and decided to punish him by using the 15 Scarlet Needles. Athena asked Milo to stop, but Kanon begged him to continue, as he thought it's only fair he got this sort of punishment. Once it was the last needle's turn, Milo actually touched the vital point of his body so that he could stop the bleeding. With this, Milo acknowledged Kanon as the new Saint of Gemini. While this happened, Kanon created an illusion by using the Gemini Cloth in order to block Shura Capricorn, Camus Aquarius and his own older brother Saga Gemini (These three were brought back to life by Hades, alongside the former Pope Sion and the other Gold Saints that died in the past). Then, once Athena called the three Saints, Kanon delivered to Saga the golden dagger that he tried to use to stab Athena when she was a child. After that, he wore the Gemini Cloth and went to the Underworld with the rest of the Saint in order to fight against Hades.
In the Underworld, Kanon is one of the Saints that defeated most of the specters and ended up being the archnemesis of Rhadamanthys Wyvern, one of the three judges of hell. In their first battle, Kanon is able to block him by using the Genro-Mao-Ken. He then tried to use him to kill Hades, but Rhadamanthys was helped by some of his soldiers, who then got defeated by Kanon. In their second battle, Kanon was able to block Rhadamanthys attack and sent him to the ground with the Galaxian Explosion. Rhadamanthys got helped then by Aiacos Garuda and Minos Griffon, the other two judges of hell, who put Kanon in trouble before he was saved by Ikki. Kanon then looked at the battle between Ikki and Aiacos Garuda, with the latter being defeated and the former disappear.
Kanon somehow managed to escape from the two judges of hell and noticed that the other Gold Cloths were calling the Gemini one. Before he could reach them, Rhadamanthys followed him. Knowing that there was no time to lose, Kanon decided to take off his Cloth, who reached the other ones. In the end, Kanon decided to sacrifice himself in order to defeat Rhadamanthys. He said that he redeemed his past and has no more regrets. With that, Kanon uses one last Galaxian Explosion to defeat Rhadamanthys and then he disappears into ashes. After that, the Gemini Cloth reached the others and Saga Gemini temporarely returned in order to destroy the Wailing Wall with the others Gold Saints.