Michael Ryan is the estranged father of Kalvin Ryan, the patient in the Season 2 episode Hunting. He was portrayed by actor Wings Hauser.
Kalvin told the team that he didn't want anyone to contact his father because he had thrown him out of the house when he found out Kalvin was gay. However, when Kalvin thought he was dying, he asked Allison Cameron to call his father. Gregory House advised against it, but Cameron did it anyway.
However, when Michael arrived, Kalvin had improved and wanted nothing to do with him. Michael felt the same way and left Kalvin's room to wait elsewhere.
As Kalvin's case progressed, Cameron tried to reconcile them, but when she accused Michael of throwing Ryan out, she found out that Michael had no problem with Kalvin being gay and hadn't tossed him out. Instead, their relationship broke apart when Kalvin's mother needed a transplant and Kalvin was disqualified as a donor because he had HIV. Otherwise, he was a perfect match. They had argued about whether Kalvin's "party boy" lifestyle was to blame.
However, House finally figured out that Michael's supposed cirrhosis might actually be the same problem that Kalvin had, only with a different presentation. He asked Michael if he had ever been hunting with Kalvin. Michael admitted they had, and House suspected they both had Echinococcosis. However, Michael refused to be tested for it, and Kalvin was too sick to be tested for it. Instead, House picked a fight with Michael and, after Michael hit him, House hit him in the liver with his cane. This set off anaphylactic shock because House had burst a parasitic cyst. This confirmed Kalvin probably had it as well and they scheduled cryosurgery for both men to safely remove the cysts.