Multiverse Explorer

Terran man befriends 2 people from another world and is invited to visit them on their world. While there, he develops a close relationship with a juvenile member of an ancient alien race who is not even remotely human, physically, but has a very human personality. Together, the two of them provide a valuable service to their hosts that leads to the Terran being hired to work as an Explorer for the other world. That job leads to an unending series of adventures and priceless friendships.