Ivan, who House consistently refers to as "Voldemort", was an orderly in House's rehab clinic in the Season 3 episode Words and Deeds. He was portrayed by actor Brian Leckner.
Ivan is primarily responsible for enforcing the clinic's rules, such as ensuring that patients do not have contriband and that they are in the right place at the right time.
However, when House is in jail serving his contempt sentence, Wilson learns that Ivan has secretly been supplying House with Vicodin instead of his medication in return for a bribe.