A 22-year-old, hormonal, blonde-haired biker and virgin, Onizuka Eikichi graduated from a bottom-rung university by cheating and, as such, cannot get a decent job. His main way of spending time is peering up girls' skirts in a local mall. He is very athletic, as he can bench press 150 kg (331 lbs), has a second dan black belt in karate, and even claims to perform 500 push-ups, 1000 pull ups and 2000 squats daily.

Through the events explained in the Great Teacher Onizuka manga, Onizuka decides to become a teacher, even though it is later implied that he has an IQ of about 50 (Though not necessarily true). His initial teacher training is in the Musashino Public High School, where he meets Nanako Mizuki. His experience taming the rowdy gangs in his assigned class hardens his convictions that teaching is the way to go, and when he learns of Mizuki's problems, he also decides to abstain from sexual experience with his students, opting to solve their personal issues for them, instead.

Unfortunately, he nearly messes up by forgetting to take the public teachers' civil service exam. As a result, no public high school will take him, but he is still eligible for a number of private schools. He manages to get a job at the upper-crust Holy Forest Academy, despite the objections of vice-principal Uchiyamada Hiroshi, whom he continues to aggravate well into his tenure. Of course, one of the conditions of having the job at Holy Forest is that he must sleep at the school—in the storage room at the top floor, with roof access—and it is here that Onizuka officially starts his career in teaching, when he stops Yoshikawa Noboru from committing suicide.

Onizuka is put in charge of class 3-4, a class so bad it has already driven several teachers insane. Not only does he survive the class's brutal bullying tactics, but he also befriends his students, and the backbone to the story of Great Teacher Onizuka consists of his unique experiences in turning his students around and learning lessons of his own.

It is later implied again, during the National Exams, that Onizuka might actually be some sort of genius. His exam results were swapped: once for full marks, and once for just short of full marks. However, when Yoshito Kikuchi is asked whether he is curious about Onizuka's true results, he checks the answers himself and the result amazes him. He checks with Onizuka, who states that he definitely got perfect results from his own effort (which would be incredible, since he did a 5 hour exam in 1 hour, bleeding, and having 4 bullets in his stomach).

Onizuka has incredible physical resilience. On more than one occasion, he has fallen from heights that would instantly kill most people and claims to have a sort of healing factor, as he healed from a broken arm in less than a day, and even endure multiple gunshot wounds, in a short period of time. Onizuka often presents his strength in unintentionally flamboyant ways, such as arm wrestling over 100 men of exponential strength in a row and winning (including the character Heihachi from the Tekken series of fighting games, and Jason Voorhees from the films called Friday the 13th (series)). His fighting abilities are not to be taken lightly either, as Onizuka is capable of fending off multiple opponents, even if they are well-armed, and almost never receives any physical damage.

In spite of his impressive fighting skills, Onizuka is often roughed up by his students, and others, whenever he behaves badly. It could be speculated that deep down, Onizuka knows when he's acting immature, and allows others to keep him in line. An example of this is when he forces his students to dig the earth for buried treasure during the Okinawa trip. Kanzaki Urumi kicks Onizuka into a hole, and later forces him to wear S&M garb and has him crawl around on all fours, with Urumi riding on his back. Yet another example is when Principal Uchiyamada has Onizuka over for dinner and hits him upside the head with a wooden bench after finding out that Onizuka was acting a bit of a letch with his daughter one day (Onizuka was playing "where's the nipple" with his daughter, at a karaoke bar in town).

His personal mode of transportation is a Kawasaki Z2 motorcycle 750cc type S. The bike itself is also featured heavily in the prequel series to GTO.