Cesare is a Shrine Master in the town of Togle and has an adopted daughter named Rio. He takes Phayne into his home in Monster Rancher 4 and encourages Rio to be his ranching assistant.
Later on in the game you learn more of Rio's origins, and realize that Cesare was entrusted with her disc after she was sealed inside by the Phoenix. Cesare then unlocked her and raised her as his own when he thought it was safe.