Yako Tenma is the main antagonist of Yu-Gi-Oh! R. He is the protégé and adopted son (often mistaken, or mistranslated as younger brother) of Pegasus, and the biological twin brother of Gekko Tenma, who wants revenge for Pegasus' death and disappearance, although unbeknownst to him, it was actually Yami Bakura who killed him. He was always known as the less superior of the two Tenma brothers, constantly being defeated by Gekko. Yako appeared to have acquired an inferiority complex from his treatment by others. Pegasus however, seemed to be the only one who recognized he had potential, and that he'd develop into a great Duelist someday. He believes that Yugi killed Pegasus, and even when Gekko tells him that what Keith Howard told him is false, he said that "the reason Pegasus got killed is because Yugi won against him." Tenma's name comes from "Pegasus", which can be translated into Japanese as "Tenma", or 天馬.
Yako along his twin brother, Gekko were orphans adopted by Pegasus to be trained as his protégés to be potential heirs to his company, I2. It became apparent the Tenma twins were considered Pegasus' favorites amongst the adopted children, and they clearly were the most attached to him. One day prior to Duelist Kingdom, Pegasus discussed his plans of taking over KaibaCorp and reviving Cyndia. Unfortunately, this dream would never come true, and it was also the last time Yako would see his mentor alive. At some point after The Duelist Kingdom Tournament, he found Cyndia's card covered in Pegasus's blood, causing Yako to freak out.
In anger, Yako demanded Mr. Crocketts tell him who did this, and not knowing what happened to Pegasus, he revealed that Yugi defeated him, giving Yako a target.
Yako is extremely determined to revive Pegasus, not letting anything stand in his way; be it Yugi and his friends or Seto Kaiba. His mentality seemed to be slowly warped by the presence of the Devil Gods, twisting him more and more as he had been exposed to them over time. He claims that Pegasus was his master and everything to him, and losing him was the worst tragedy that could befall him.
He also held great enmity for his twin, Gekko. Gekko was often praised by Pegasus as a "perfect" duelist, and Yako often felt that he was dueling in the shadow of his twin. He failed to realize that Gekko was "perfect" in the fact that he would experience no growth as a duelist, and was as strong as he'd ever be. Yako's feelings for Gekko eventually caused him to use his brother as a pawn in his battle against Yugi.
After the destruction of The Wicked Avatar, Yako's personality returned to his kind, calm demeanor he held before being exposed to the gods.
He is highly skilled with technology, using holograms to make it appear as if he could pass through walls, ceilings and other solid objects.