Saejima was the leader of his own biker gang back in the day, they called him the「Mad dog of Kamukura」; now, he’s a cop… a transition few understand.
Despite being a cop, he is not unwilling to commit crimes, having several side-business’ that frequently dip into the illegal. Despite this, he is a good man at heart and his own spin-off shows him become better. He deeply regrets many of his past actions.
He is, like Ryuji, also quite loyal to Onizuka… willing to do all manner of things to help him out… although he does frequently try to double-cross or trick Onizuka. However, he does help out Onizuka when he realises he really needs help. When Teshigawara starts rampaging, he helps Onizuka get to him and he’s even willing to kill Teshigawara if Onizuka told him to, saying he’s willing to cover it up. While not the most legally-conscious, he is still ultimately loyal to his friends.