Eikichi Onizuka

Eikichi Onizuka is the main character of the anime Great Teacher Onizuka

Background
Eikich Onizuka is the former head of a now defunct biker gang known as the Onibaku sometime after college he did decide to become a teacher which as he said was 'The Girls' he hoped to make girls fall for him but his meeting with student Nanako Mizuki (who at first tried to partake in a blackmail scheme with delinquents) he helped her with her parents and his actions which did include bashing a hole in a wall with a sledge hammer did get Nanako's parents to notice each other but it did inspire Nanako to try to 'destroy the wall between them in their hearts' and Onizuka really earned Nanako's affection (though she was not seen in the anime again).

But a bigger challenge awaited Onizuka at Holy Forest Academy where he was eventually hired by the Dean Ryoko Sakurai not just in times of Vice Principal Hiroshi Uchiyamiyada who strongly disliked him but the students in his homeroom class who now despised teachers (he would learn of the tragedy that turned them against teachers much later in the anime).

Onizuka began winning his students over by helping them or through other means but at the end of the anime he was forced to make a choice....

Abilities
Onizuka is shown to be immensely strong and physically able plus he has excellent fighting abilities from his years on the street he has a 2nd Degree Black Belt in Karate (possibly a full contact style) and is also adept at wrestling.

Personality
Eikichi Onizuka unlike in the manga was more a good guy while he can be brash, perverted, bit crude and there have been occassions where his attempts to show sincerity border in ridiculous but he in the anime he was far more believable as a teacher, more sympathetic, more human and was far more true to himself.

Onizuka was friendly to his students he did his best at all times for them to help their problems but unlike his manga portrayal did understand mostly when to stand back and let his students make their own mistakes also his caring and his faith in them is undeniable unlike the manga portrayal which is very questionable at best.

Onizuka was a pervert but a likeable he flirted with fellow teacher Azusa Fuyutsuki but it was very low key but he did show attraction to some of his students most obvious one was Tomoko Nomura (at one time Tomoko offered to let Onizuka touch her breasts but Onizuka declined if she weren't his student he'd in all likelihood have taken her up).

Onizuka was not afraid to call people out on their hypocrisies including his students.

Also Onizuka did show practicality and common sense when he disciplined in an extreme manner but when he talked to Ryuji he did show nervousness but also a fair amount of remorse and concern that the Minors Bureau of the Japanese Police might come after him.

When it came to something to extreme it was very, very, very, very rare and as a last resort when he drove Urumi off the bridge on his bike it was more like he knew if he did it any other way Vice Principal Uchiyamiyada would have interfered.

Also Onizuka was willing to make sacrifices on himself and it was made clear at the end of the anime.