Giratina gijinka OC.
Her name's Remnant, and she was fused with Giratina by scientists. Now Giratina's body of antimatter is eating her from the inside out, so she's dying for incurable cancer, basically. But she's very loving and sweet and nice, as well as powerful and slightly vengeful. And she believes her purpose is to be an angel, to love the living and guide the dying. She's, yes, a mercy killer.