You are right to indicate only two characters (Michael and Hannah) because our narrator is so engrossed in the one relationship, he has no time to include any other characters (even his own daughter)!
Let's begin with Michael. He is our completely unreliable narrator. He admits that he doesn't have a good memory. Take a look at this quotation:
My bundle of memories is so small. Or do I keep it small? I wonder if my memory of happiness is even true. If I think about it more, plenty of embarrassing and painful situations come to mind, and I know that even if I had said goodbye to the memory of Hanna, I had not overcome it.
As you can see, Michael nicely symbolizes the Germany that chooses to forget Nazi secrets of the past. Michael is greatly affected by his relationship with Hanna, and especially by their many trysts as he recuperates from hepatitis. Where Michael used to be quite studious, now he is happy to skip class to see Hanna. Michael feels guilt in making new friends at the pool while he is seeing Hanna. Ironically, Michael doesn't acknowledge Hanna when she does show up at the pool. It is here that Hanna vanishes from Michael's life until her trial later. Michael is a bit aimless after Hanna, marrying and having a child without giving them much thought. It is only when he begins sending her tapes in prison that the story really begins again. Michael now likes the distance between the two and is a bit unnerved when Hanna is scheduled for release and asks to meet with him. She is a shadow of the woman he once knew and hangs herself before her exit from prison. The two characters think differently about many things. Take, for example, Hanna's response to Michael complaining about his work in order to go to bed with her:
"Out." She threw back the blanket. "Get out of my bed. And if you don't want to do your work, don't come back. Your work is idiotic? Idiotic? What do you think selling and punching tickets is"
Such is a perfect transition into the character of Hanna. Hanna is absolutely insistent that Michael keep up with his school work if he wants to keep seeing her. Little does he know, at this point, that Hanna is unable to read. This is the reason why she feels studies are so important. She is also a fastidious cleaner and very exacting, almost rude. In this way she is often seen as a symbol of the "hidden" past of Nazi Germany. Hanna does finally learn to read and write in prison; however, she is obviously so affected by Michael's disappointment in how she currently looks that she hangs herself. Interesting that her dying wish is to make reparations (with all her savings) to the only survivor of the church Hanna supposedly burned.