Umibōzu (means "sea bonze", which is a ghost in Japanese mythology; real name: Hayato Ijuin), is a "sweeper" in the underground just like Ryo Saeba. With his giant body, and his superb combat skills, he doesn't really need a partner. He is Saeba's equal in most things, and the two have somewhat of a rivalry going.
Umibozu fought against Ryo back in Central America, and had his eyes damaged by Saeba. This should make them enemies, but both men shrug off the past since what happened in the war was not personal. That doesn't mean that they don't fight, though. They often find each other on the same case, but on different sides, and have dueled more than once.