Killer B is the jinchuriki of the Eight-Tails and a ninja from the Hidden Cloud Village. Killer B serves as one of the main protagonists in Super Smash Bros. Crossover starting with Season 20.
Not much is known about Killer B's life before the events of Super Smash Bros. Crossover, other than his status as the jinchuriki of the Eight-Tails.
Killer B was minding his own business when the Akatsuki attacked his village. At the same time, Magneto arrived, planning on taking Killer B to safety by force if necessary. Killer B then looked on as Mag Mel and Evolved Razenoid gave up their lives in a vain effort to destroy Madara Uchiha, and then witnessed Evo Phantom Dharak mutate into Razenoid. After the Akatsuki and Magneto's forces retreated, Killer B was teleported to the Hidden Leaf Village and then finally at Smash City. Despite the confusion from what happened, Killer B is willing to work with the heroes to stop the Akatsuki. Killer B later has the Eight-Tails extracted from him, but is revived before the final battle against Samus Aran begins.