Yue Katou

Sakuya Kira's drug-dealing best friend - killed when Setsuna (under Alexiel's control) is tricked by Rosiel.
When he dies, Enra-Ou enlists his help to test (and preferably kill) the Messiah. While he and Setsuna do not get along initially, they become friends and defeat Enra-Ou. He acts as a guide to Setsuna while he is in Hades and dies in order for them to defeat Enra-Ou. He is then revived and given a new body by Uriel, going on to help Setsuna in the remaining story arcs. Initially a source of comic relief, Yue slowly becomes a tragic character as his new body begins to decay, particularly after being grievously hurt by Lucifer.