Sei is a nineteen-year-old boy and protagonist of Suicide Island (Jisatsutou).

At the age of 19, Sei, a suicidal teenager, cuts his left wrist and administers a medicinal drug overdose to himself. He has an entry in the "suicidal recidivist" list of the government and signs a document upon his expected death in the hospital, after which he is left with no rights and finds himself on an unknown island, a place which is subject to no law.