Nack works so well as a comparison to Usato, 2 people who made it through that absurd training for very different reasons. Usato made it due to how much of a stubborn dickhead he was, and that he had the will to never give up. Nack made it because it was the first time he had ever felt that his healing magic was useful. He spent his life being told he was a failure because of his magic, and was partially disowned because of it. But there was a way for him to become needed because of his magic. It's a no-brainer from his pov