Spy against Blackbeard or do you genuinely believe that he truly wanted to be a pirate? It doesn't take a genius to understand that Kuzan has ulterior motives in joining the crew - otherwise, Oda would include Janitoryu into the pirate king race game and not Kuzan, lol.
Who is he secretly working for?
Garp, he and Shiryu retired Garp. Dragon, nah not making sense the 2 haven't interacted on screen.
Akainu, nah he left because of Akainu.
Kuzan is loyal to Blackbeard. He might have a goal on his own but that goal flows together with Blackbeard's goal.