How does Michael "betray" Hanna, as he explains in Chapter 15?

At first Michael says that he doesn't acknowledge Hanna's existence to others. Michael does, however, admit that he is keeping things from other people, or as he puts it, "disowning [Hanna]." Michael doesn't tell his friends about Hanna. Michael doesn't tell Sophie about Hanna, even when he has a chance, and in that way, he is betraying her.