He is Ryoko Sakurai's (Seirin's Principal) grandson and appears only in the manga.
Mayu Wakui (和久井繭 lit. Wakui Mayu) comes from a very rich family and for an unknown reason he kinda abandoned the school from a long time, becoming a delinquent. Despite this he's still enrolled in his grandmother's school but doesn't attend it almost at all.
Introduction
He is the leader of a motorcycle gang composed of delinquent students like himself. Unlike most of the rebel students, Mayu has an army behind him. His gang has 100 members.
Appearance
Mayu is very athletic with his skateboard and is good in fights (his kicks are really strong), in fact he is very well known as being the toughest student in the school.
He is sort of an idol due mainly to his extreme beauty.
The boy can be considered the male equivalent to Urumi Kanzaki. He looks similar to Urumi in terms of physical appearance and also has an IQ above 200 like the girl.
History
Mayu appears first in volume 14, giving help to Miyabi in order to get Onizuka fired; although he will end up becoming a great friend of his, especially enjoying how Onizuka will be able to constantly reply to his jokes.
He then went on to become a singer, under the same management as Tomoko Nomura.
Mayu suffers from a terminal illness because of a medical mistreatment and later makes a debut in the mainstream media by confessing his illness. This is the reason to his chaotic manner as no matter what he does, he can be dead the next day. Due to this limited life-span knowledge he seems to have become a drug addict. It's not clear if it's the drug usage or his mysterious illness (probably both), but both will eventually kill him.