What hurts Risa most of all about being unwound?

The answer to this question can be found near the end of chapter 2. This is the first chapter that is about Risa in the book.

Mr. Durkin does not come. This hurts Risa most of all.

Mr. Durkin is Risa's piano teacher, and she loves him as a teacher. In fact, readers are told that Mr. Durkin has become so much more than a teacher to her over the last five years. Mr. Durkin is the closest thing that Risa has to a father, and that is a big deal to a girl that has grown up as an orphan.

Mr. Durkin is out there. He has been her piano teacher for five years. He's the closest thing Risa has to a parent. She's lucky. Not every kid at Ohio State Home 23 has a teacher they can say that about. Most StaHo kids hate their teachers, because they see them as jailers.

The chapter begins with Risa playing piano, and she makes just a few mistakes in the piece that she is playing. Mr. Durkin assures Risa that it is no big deal that she shouldn't worry about it. Unfortunately, Risa's fears about her mistakes are real. She hasn't quite progressed far enough to get herself adopted, and the State Home needs to make room for new kids; therefore, Risa is scheduled to be unwound. As she is being escorted out, Risa realizes that Mr. Durkin didn't come to see her one last time. That is a brutal blow to Risa, because she had come to trust and love him. It hurts her that he has cast her aside so quickly and easily.

What hurts Risa most of all about being unwound is that she feels betrayed by Mr. Durkin, her piano teacher. She always thought she could rely on him. Indeed, he was the only adult on whom she could rely. After Risa is slated to be unwound, a number of her school friends sneak off to bid her farewell. But Mr. Durkin isn't among them, and his failure to put in an appearance makes Risa very upset indeed.
Durkin had always been a mentor to Risa; more than that, he was almost a father figure. But it now seems to Risa that Mr. Durkin is just like all the other adults in her life. His refusal to bid her farewell before she's shipped off to be unwound, his betrayal of her, symbolizes how children in this dystopian society are treated by those who are supposed to love, protect, and care for them.