Ikki was one of the 100 fatherless children of Mitsumasa Kido, sent to train in search of a cloth, to become the saints of Athena; different from the other 98 orphans, Ikki was Shun's brother, both fathers as the mother. In the anime, how he doesn't a Kido son, he has another goal (the Pope sent him to recover the Sagittarius cloth, but, hedoesn't serve Athena).
After much time training in Graad foundation, it comes time to decide where it goes each orphan. Shun by chance, it would be forwarded to the Death Queen Island, Hell on Earth; Ikki intervenes to prevent his younger brother died, and decides to go in his place. But only after Shun from, Ikki is warned that his brother went to Andromeda Island, as hellish place like Death Queen Island, where you will surely die. But now that Shun is gone, there's no way to intervene.
Origins of Evil
On the Death Queen Island, Ikki meets a slave girl that has a similar look with his brother (only female), with whom he falls in love; however, during their training, this slave dies, and Ikki arouses hatred in his heart, forgetting his humble nature and preached protection of his brothers. The Ikki master tells him that his father was Mitsumasa Kido, who rejected the tutelage of their children and put them at the orphanage, to turn holy and promote the Galactic Wars. But as Mitsumasa died during the period when Ikki trained, you cannot cash their hatred who won, and so decides to kill everyone that is connected to Kido, as Saori, Seiya, your butler and even his brother more youth: Shun.
Ikki takes the Phoenix Cloth and goes to the headquarters of the Galactic Wars, Japan, stealing the Sagittarius Cloth, the tournament prize, causing its cancellation. He plans to use to gain power; but first, he wants revenge on the descendants of Kido.
Ikki ends up being defeated by Seiya and the other saints, and dies in the Mount Fuji earthquake location you had called the saints to face him. Ikki then revive as a servant of Athena, and admits that to be the weakest of all (emotionally), he thought he lived Hell, but all lived bad things during your workouts. Phoenix, will now be again this humble man who protects Shun and his other half-brothers. He dies in the destruction of Mount Fuji, but revives to save his half-brothers.
Ikki is similar to Darth Vader: he fell into evil after being trained by an evil master (Guilty for Ikki,Palpatine Vader) and have lost someone dear to him (Esmeralda for Ikki, Padme for Vader) and, after killing his teacher, can redeem himself.