Bleach

Haineko Cries! The Tragic Sword Beast (13x33)

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While on a mission with Rangiku to fight sword fiends, Haineko encounters one named Narunosuke, whose master was a coward, and who went with the others in order to become stronger. Haineko becomes friends with Narunosuke, sneaking him food and medicine despite Rangiku and the other Soul Reapers' belief that the sword fiends inevitably lose control of themselves and Narunosuke's gradual loss of control. Rangiku arrives to deal with Narunosuke and Haineko tries to stop her, but when Narunosuke loses control of himself, Haineko is ultimately forced to kill him.