The Bartender appeared both in House's hallucinations and real life experiences. When House was trying to remember who he saw who was sick in House's Head, he remembered being at a bar and that someone was drinking sherry. He remembered the Bartender took his keys, which is why he was on the bus.
House finally realizes that Amber was on the bus with him and they find her, in critical condition, at Princeton General. House can't remember why he was with Amber. Kutner remembered a "Sharrie's Bar" near the bus crash site and they went to investigate. The Bartender recognized him and returned House's keys. The Bartender also remembered House was with Amber, and describes her as House's "girlfriend" even though House denies she is. The only diagnostic clue he can provide is that Amber was sneezing. House finally remembered that when the bartender took away his keys, he called Wilson to pick him up, but Amber got the message instead. Amber came to meet him.