Kanon and Saga, the Gemini twins of Saint Seiya.
One of them, Kanon, is evil incarnate, while the other, Saga, is a great sage and warrior loved by the people and destined to perform great deeds. Because Kanon is pure malice, Saga has no other choice but to incarcerate him.
But the damage has been done. Kanon has managed to corrupt Saga and create an evil persona in him. As a result, instead of great deeds, Saga usurps the position of Great Patriarch and kills his best friend, who knew the secret.
Saga's evil rule is put to an end at the Battle of the 12 Houses thanks to the bronze saints, after most of the gold saints have died. He apologizes to Athena and we can see an evil spirit leaving his body (which has led to fans suggesting that maybe he was possessed by an evil entity, most probably Ares, and that he wasn't truly evil) and dies by suicide.
Meanwhile Kanon, now free, manages to completely fool a god like Poseidon and makes him his puppet. But once confronted by Phoenix Ikki, he's defeated and for once he understands what makes the Saints so great. With Poseidon fully awoke, he sacrifices himself...
But that isn't the end of his redemption. Somehow he survives and after being tortured by Scorpio (as the gold Saints hadn't forgiven him) to a mortyfing degree to test his true determination, at last the Gemini armor has a worthy wearer and he goes into battle, defeating most of Hades's army by himself and allowing the bronze saints to move freely and undetected in the Underworld while he drives up the attention upon himself. He develops a rivalry with one of Hades's three top generals, which ends up with him giving back the armor to his now resurrected brother for a final attack at Hades's last barrier, and he sacrifices once again (and this time, for good) to keep his rival in check, killing him and himself with a colossal Galactic Explosion (the Gemini signature attack) that goes into the stars.
Saint Seiya: Lost Canvas also has a tragic pair of Geminis twins, but I know little about them.