Corptain in Yo-kai Watch 2

After completing the main story, you will find a quest near the entrance to the Mt. Wildwood Sacred Tree Path. This will unlock the Yo-kai World where you meet Corptain and complete his request of stopping the fights in the 3 areas. After doing so, you will need to defeat the leaders in the 3rd area of the realm. After doing so, Corptain will be able to be battled and befriended each day through usual methods.