One of Baelgard's four guardians. Originally a normal and kind elf, until he was infected by the darkness. Though he is the brother of Astraea's queen, Celestia, he kept Xecty from knowing her true origins. He's shown to possess immense dark energy, as shown after Xecty's death, when he released said energy as a powerful blast, which covered nearly all of Liberia in an icy blizzard. His anger prompted him to blame all living souls, and after controlling the dark elven army of Baelgard, he plans to destroy everything. Only after Souma, Kiriya, and Saionji use the combined powers of their Holy Grails, that his soul is liberated. Together with his sister Celestia, he decides to stay behind and be sealed in the other dimension.

His soul blade is called Demon Gun Sword Gallatin (魔砲剣　ガラティン, Mahōken Garatin?), a giant sword able to shoot laser beams. Unlike the others, Killrain himself transforms into a giant sword, rather than having the sword drawn from within him.