Itoshiki Nozomu is the main character of Sayonara Zetsubou Sensei and the teacher of a class of 32 students. His name contains an unlucky number of characters, and when it is written horizontally, his name forms the kanji for zetsubou (meaning despair, which is befitting of his pessimistic and depressive personality). He has a tendency to attempt to commit suicide (usually involving a noose hanging from a tree) for a number of often bizarre reasons, though he miraculously manages to survive every attempt. His catch phrase is "Zetsubou shita!" ("I'm in despair!"), which is declared in many of his rantings throughout the series. Most of the main girls seem to have feelings for him but only because they misunderstand what he said to each of them (during the episodes introducing them). His negative personality started blossoming when he was in his first year of high school when he 'accidentally' joined a 'Negative' club. He is based upon the main character from Dazai Osamu's No Longer Human, which is both Nozomu's and the author's favorite novel.
Other Itoshiki family members have similar puns based on their names.