I feel Sgt. Burly from Yokai Watch would make a nice honorary third member.
He's an Oni like Gargaros, but while most oni are scary and try to terrorize children into good behavior, Sg. Burly is a charismatic coach that encourages children to exercice. Being a spitir, anyone he posesses also becomes obsessed with getting swole.
https://www.youtube.com/watch?v=YP3EPySPmo4