In, Under My Skin, House told Wilson that he was hallucinating but he did not want to tell his that he was seeing Amber because she was Wilson's ex-girlfriend. So, he tried to convince him that he was seeing Kutner instead.
In Both Sides Now, when House realizes he never detoxed nor slept with Cuddy, he sees Amber and a new Kutner Hallucination, showing that his psychosis is mostly consistent of dead people. After Amber says "So this is the story you made up about who you are. It's a nice one," to which a Kutner Hallucination adds "Too bad it isn't true.". After saying that both hallucinations disappear.
In Everybody Dies, Kutner is the first hallucination that appears to House in the burning warehouse where House has taken heroin. House has a dialogue with the hallucination for several minutes before it is replaced by the Amber hallucination.