Mayu Wakui is a student belonging to Onizuka's homeroom class, Class 4. Though enrolled in Onizuka's class, he does not come to school regularly. Mayu is the grandson of Ryoko Sakurai, the headmistress of Holy Forest Academy. most are unaware of this fact. He is widely known within Holy Forest as being the toughest student in the school. He is also known for being the most rebellious. Mayu suffers from a mysterious illness which will eventually kill him. What caused him to go bad is not fully explained but it can be assumed that he went bad because he figured he was going to die anyway.

Mayu can be considered the male equivalent to Urumi Kanzaki. He looks very similar to her. He is equal to her in terms of intelligence. He also does classroom terrorism. He has proven to be just as cunning as Kanzaki on occasion. On an interesting note, Mayu seems to dislike Kanzaki despite the fact that they are so similar.

In the past, it is shown that Mayu and Kikuchi were very close. Kikuchi seems to be the only one who knows the secret of Mayu's illness. Fujiyoshi also mentions that Mayu used to be a shy, quiet student. While this is never explored, it can be assumed that Mayu was once a good stduent.

Mayu has girlfriend named Ageha, who is a stripper at a club called A-10. He is also the leader of a motorcycle gang. Mayu comes from an upper middle class family. His parents are always worrying about him. His grandmother in particular worries about him a lot.