Yamu is a smaller version of Spopovich. He's bald, muscular, and wears black pants. Like Spopovich, Yamu has a mark of a dark letter "M" on his forehead.
Though it was never mentioned, it is likely that Yamu easily lost during one of the Worlds Martial Arts Tournament and was granted by Babidi the powers of Majin.
25th World Martial Arts Tournament
Yamu and Spopovich entered the 25th World Martial Arts Tournament to find energy to give to Majin Buu. When Spopovich was beating up Videl, Yamu stopped him from killing Videl, as gathering energy was more important than fighting in the tournament.
Stealing Gohan's Power
When Gohan powered up to Super Saiyan 2, Yamu used an energy absorbing device to drain Gohan's energy. He and Spopovich left to take the energy to Babidi.
After Babidi killed Spopovich, Yamu tried to escape, but Pui Pui destroyed him with an energy wave.
Yamu is far more intelligent than Spopovich. He also show some sense of honor when he told Spopovich not to kill Videl and that he already won the match. However, Yamu is also a cheater. When Spopovich and Gohan were fighting each other, Yamu steps in, joins Spopovich, and drains Gohan's power when he was in his Super Saiyan 2 form.
Like nearly every villain in the Dragon Ball Series, Yamu can fly, generate Ki energy attacks, and has superhuman strength. He also wields a weapon that drains opponent's Power Level called the Energy Absorber.