Dromp in Yo-kai Watch 1

Dromp can be befriended by completing the post-game quest Flower For The Shrine, given by one of the receptionists at Gourd Pond Museum. First collect a Forever Flower from white Pinkipoos in the Yo-kai World, then go the far right end of the section of the Abandoned Tunnel behind the rank A Watch Lock. Fight the Signiton (if you haven't already), then keep going until you're back outside and reach the shrine behind the waterfall to the left. Place the flower on the shrine, and Dromp will appear to fight you, after which he'll automatically befriend you.