Zerberker in Yo-kai Watch 1

One way to befriend Zerberker is to complete the post-game quest An Offered Sword, which is given by the old lady visiting the Fortune Hospital in Downtown Springdale during daytime. Obtain a Betrayed Blade by defeating White Reuknights at Hooligan Road in the Yo-kai World, then go to the graveyard behind the Shoten Temple at night, and give Zerberker his lost sword. He will then fight the player and join the player automatically once they defeat him.

Alternatively, Zerberker appears in the 7th Circle of the Infinite Inferno.