The titular character of the series, the Great Teacher himself.
Prior to GTO, he was one of the main characters of ｢Shonan Junai-gumi｣ where he formed and led a biker gang. At the start of GTO, he is no longer in a biker gang and he works odd jobs, not really knowing what he wants to do with his life. After an encounter with a hot highschool girl (and learning she was in a relationship with her teacher), he decides to become a teacher. After becoming a teacher though, he finds he actually loves the job, living up to his (self-appointed) nickname.
His personality can be best described as simple. He thinks very highly of himself (for those who couldn’t tell from Great Teacher Onizuka), but he keeps it within reasonable boundaries… so he’s more confident than egotistical. Onizuka doesn’t tend to think things through and lets his own feelings get the better of him (except when he realises he needs to be calm), this can lead to both hilarious and horrifying results. As a result, he gets angered easily, very basic seduction techniques always get him and his greed… well yeah. He also occasionally dresses up, just for fun, seemingly. His body is also that of a god as he can do the impossible and survive the impossible (he fell off the roof of the school, landing on a car and came out with light bruises), leading other characters to describe him as a cockroach.
However, one aspect of his character is as clear as day: his undying loyalty to his students. Onizuka would die if it meant helping his students. This likely stems from his undying loyalty to those he considers his friends (he essentially views his students as his friends and treats them as such). This probably makes him one of the greatest teachers ever.
His teaching style isn’t based around raising student test scores (he doesn’t even understand the subject he’s meant to teach), but about making them into decent human beings, true to their own principles. He frequently clashes with the rest of the teaching staff over his extremely unorthodox teaching style (which occasionally looks to threaten the lives of his own students). What makes him such a great teacher is how he treats the students as equals… they insult him, he insults them back. The students don’t see him as a teacher, they see him as a trusted friend.
And, truly, is that not beautiful?