An eyebrow-less boxing champion who tricks Eikichi with a knuckle-duster and evasive maneuvers- he finally wins by smashing Eikichi with a street sign. While a reasonably good fighter, he only won because Eikichi wasn't expecting him to be trained- the next time they cross paths, Ryuji takes down the whole gang singlehanded while Eikichi beats Machida's teeth out, taking the boxer's punches and hitting him back harder.

(Source: Wikipedia)

Before Eikichi Onizuka would become the "greatest teacher in the world," he and Ryuji Danma were members of the infamous biker gang, Oni Baku. When they weren't out riding around and getting into trouble, this duo could be found in school. Doing what you ask? Picking up young women of course! 
(Source: Tokyopop)

Eikichi Onizuka and Ryuji Danma are members of infamous biker duo, the Oni Baku. When not out riding around, they can be found in school, trying to pick up young women. This is the story of the young Onizuka, who would later become the greatest teacher in Japan, and his partner Ryuji in their quest to lose their virginity and reach maturity. 
They are widely feared bosozoku, and are known for their tenacity and viciousness in a fight. However, this lifestyle does not exactly endear them to the opposite gender, so they decide to change their ways. However, this is easier said than done. 
(Source: Anime News Network)