Some TMNT stuff really isn't for little kids.
Jei is an undying anthropomorphic wolf who appears in Usagi Yojimbo. He is often considered Usagi's true arch-nemesis. Jei believes that he is the "blade of the gods" - a samurai tasked by the deities to smite sinners. The problem is, any "sin" no matter how small, qualifies in Jei's mind. This has brought him to clash with Usagi many times.
Jei is able to resurrect and possess other bodies, making him essentially immortal.
In TMNT comics
Jei is the central villain in the IDW crossover comic, Namazu. Believing that there would be no more evil if Usagi's entire land was destroyed, he enlisted a large group of bandits to stop Kakera from repairing the stone known as Kanameishi. However, Kakera had enlisted Usagi and the Turtles to accompany him on his quest.
As a result, Jei's forces attacked and were quickly repelled, but Jei himself was a formidable opponent for both Leonardo and Usagi. He defeated them both in combat and approached Kakera in order to stab him, but the rat finished the spell to repair the stone just in the nick of time. Jei was vaporized by the eruption of fire from Kanameishi.