Onizuka used to be Yankee, a japanese delinquent. His best friend is Danma Ryuji and with him he's part of the Onibaku Duo ( Oni for Onizuka and Baku for Bakudan which means Bomb.) Back in his younger days, Onizuka used to have a lot of street fights with people trying to be better than him. As the Onibaku was reputed unbeatable, his opponents were not above Kick the Dog and Moral Event Horizon on him and his friends. Bad luck for them, Onizuka used to get a little Ax-Crazy when pushed too much. He then left Shonan faking his death in front of his friends. He then moved to Tokyo. Since then, Onizuka became a teacher and softened up a bit.
Bash Brothers: With Damna Ryuji, in Shonan Jun'ai Gumi/GTO Early Years.
Catchphrase: "Onizuka Eikichi, 22 years old." before declaring his motivations to do something.
Chivalrous Pervert: He had plenty of occasions to have his way with some female students (like Urumi), but he still prefers to have consensual sex with somebody that really means something to him.
Determinator: And how! Could easily turn into a Juggernaut. Heck, you can throw literally everything at this guy and he still wont be stopped. Not even a broken clotted blood vessel in his brain could keep him down for long.
Dye Hard: His bleached hair testify of what he used to be and are usually considered improper for a teacher.
Jerk with a Heart of Gold: Onizuka's teaching methods (particularly moral lessons)are nothing short of unorthodox to put it one way and he almost always stirs up controversy wherever he goes, but unlike most teachers, Onizuka cares deeply about the welfare of his students and will see to it that they become better people under his watchful eye.
Best friend of Onizuka and the other member of the Onibaku Duo. Met Eikichi in High School, they started as enemies when Danma saw Onizuka's Ax-Crazy self beating someone to a pulp. He then wanted to anger Onizuka enough to see it again and fight him. They became friends since and he used to be The Lancer to Onizuka.After finding a girlfriend and moving to Tokyo, he took his family's bike business.
Bad Ass: He used to team up with Onizuka, so he's bound to be.
Demoted to Extra: in Early Years, he was a main character. Since GTO, he just makes cameos.
Stepford Smiler: Initially behaves this way all the time towards her students in class, thinking it to be the only way to be a proper teacher, even as her female students hurl insults toward her. She eventually learns this is only hurting her, and that it's best to just open up and be herself.
Tsundere: B-type towards Onizuka. And he likes it!
Former Rival of Onizuka during his delinquent times, Saejima was the kind of guy who would do anything to accomplish his goals, which was beating the Onibaku.He managed to force a good friend of Onizuka betray him but he was defeated anyway by Onizuka himself. 5 years later he became a cop. He makes cameos in GTO.
Anti-Hero: Type V. He is possibly the most corrupt and violent cop that can be found in any manga. However, at the end of day, he is still one of Onizuka's best, most trusted and loyal friends. If Onizuka somehow has to call for his help, he would be by his side until the end
Ax-Crazy: Like a good deal of the Early Years villains, he was.
Still is. In fact, if you somehow manage to push Onizuka to the point of running to this guy for help, you are basically fucked. God has mercy, Onizuka knows mercy, HE DOESN'T!
Bad Ass: Being one of Onizuka's partners in crime in their early years, he kicks ass!
The Runner Up Takes It All: Subverted. She was the true winner in the contest held in the anime, but only because all the postcards that were sent to her had her names covered up. In the manga, she wins this clean.
All Love Is Unrequited In Shonan 14 Days, she describes her affection of Onizuka as this because he only views her as a middle-schooler.
Humble Pie: Onizuka's approach to punish her, Naoko and Mayuko for bullying and humiliating Noboru involves tying them by their hands, spanking them (in the anime only) and take pictures of them butt-naked after letting Noburu write apologetic words on their ass (on their panties in the anime). He then uses said pictures to bargain them into giving up the ones they did of Noburu. Needless to say, this severely damages her pride.
Aesop Amnesia: Late in the manga, around the end of the Teshigawara arc, he realizes that Onizuka really is a good teacher and he's the one who's failed as an educator...but he's back to his old self around the start of the Daimon arc.