My current five main characters would probably react like this:
Cinna: Did I overdose on something?
Guy: (says nothing but begins obsessively stalking Julian from then on.)
Becka: Am I dreaming? Please tell me I don't have to visit a therapist or something?
Pom: What the ****!
Guy and bST would be the only ones to not show much surprise at such a revelation. bST because he's a literal disembodied personification of a house, so no surprise there, and Guy because he comes across reality warping creatures quite often.