Mamoru Aoyama was an inmate at the juvenile detention center. Often picked on by the other inmates due to his short stature and feeble nature, he received some training from Danpei, who teaches him dodging techniques that make him a hard opponent to fight against, even managing to score a win against one of the toughest inmates during the juvie boxing tournament.
In the fight with Joe at the beginning of the fight Joe was very confident but Aoyama hit him and Joe only resisted while everyone supported Aoyama and made fun of Joe until after several falls Joe took a guard position and Aoyama was afraid and he didn't want to hit him since he had never seen a guard position only he used it, after that aoyama finally attacks Joe but he couldn't hit Joe since Joe was moving from side to side finally using footwork and so on. beating Aoyama.
He was winning the fight against Joe But there Joe learned to use the guard so Joe used a Turtle” Guard Stance and beat Aoyama.
It was later revealed that Danpei was using Aoyama to teach Joe his guard lesson.
