Kekkaishi - Season 1 Episode 47 Destiny's Conclusion

Heisuke, along with Kagumi, corners Byaku. It is revealed that Kagumi was Byaku's dead wife and that Byaku, though no apparent, is actually nine years older than Heisuke. Several flashback scenes occur as Byaku and Heisuke chat and fight (Kagumi does all the fighting for Heisuke). Byaku performed experiments on his wife and in the end she died. This traumatized Heisuke in the past because he loved Kagumi. The Kagumi with Heisuke now is actually a demon he made a pact with. Kagumi deals a crushing blow to Byaku's chest and disables in on the ground. Heisuke walks up to Byaku and asks if he ever loved Kagumi. Upon hearing "I have never loved anyone" from a disabled Byaku, Heisuke uses his cane to end Byaku's life.