He becomes very attached to Kanade, even asking her if he can address her by her given name. He works with Kanade in order to help people move on from the afterlife.
He later confesses his love to Kanade, and finds out that it was his heart that saved her life when he donated his organs. Otonashi is the final member of the SSS to pass on. He appears after the final end credits, where he reaches out for Kanade in the real world.
When he was alive he spent most of his time with his sister in a hospital instead of studying and making friends. After his sister's death he decides to focus on his school work to become a doctor, but on his way to his college he was in a train crash. He and several other people managed to survive the crash itself, but they were sealed inside the tunnel due to a collapse.After 7 days he decides to donate his organs so he can help others before he dies from his injuries.