Callas

Callas is the main antagonist of Kamen Rider: Battride War.

History
Callas was originally a Foundation X scientist named Susumu Karashima along with his wife Rie Kanai. However, he and his wife were trapped in a bird cage-like dimension named Mnemosyne after an experiment-gone-wrong and were transformed into spirit entities, with Karashima becoming a dark spirit entity named Callas while Kanai became a light spirit entity named Canaria. Callas summoned the Riders to Mnemosyne and wiped their memories, hoping to create a world free from evil by wiping the minds of everyone. However, after the Riders manage to regain their memories with help from Canaria they defeat his forces. Callas then assumes the form of Kamen Rider Decade Violent Emotion and battles Decade, summoning Kamen Riders Eternal, Skull and New Den-O to back him up.

Though their power is same, Decade received help from Wizard and told him to destroy the Mnemosyne. Callas attempts to defend the Mnemosyne, but Canaria stops him even by entering the cage, allowing Wizard to defeat him and destroy the Mnemosyne. Callas survives the attack but rather than continue fighting he becomes an energy ball and leaves with his former wife.