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 Wicked 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 Wicked 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.
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. Crockett tell him who did this, and not knowing what happened to Pegasus, he revealed that Yugi defeated him, giving Yako a target. In revenge, Yako locked himself in his room to research the mystic science, which includes the Millennium Items, especially the Millennium Eye, and the designs of very powerful and evil cards that Pegasus left behind, but did not produce due to fear; Yako used his researches to complete them, and named them the Wicked Gods, which slowly corrupted him.