Tsukihito Amanuma, code named "Gamemaster", is one of the Sensui Seven and a major antagonist in the "Chapter Black" arc of Yu Yu Hakusho.
Amanuma grew up being neglected by his parents, who spent most of their time working and arguing with each other, and didn't have any friends. Thus, he spent most of his time playing video games. He was found one day at an arcade by Sensui, who challenged him to a round of the game he was playing. Amanuma accepted and ended up in a draw with Sensui, impressing Amanuma as he was the only person to have ever tied with him. Sensui then offered Tsukihito a chance to get back at everyone by assisting him and his followers in a plan to open a portal to the demon world to kill all of humanity, neglecting to tell him he would die also.
Amanuma was tasked with bringing the game Goblin City to life in order to halt of Team Urameshi from reaching Sensui. In order to win the game, Yusuke and his friends would have to win four times out of seven chances. After Seaman won the tennis game, Genkai won the Battle Heli and Yusuke won the King of the Hill, they had to fight Gamemaster for the fourth win.
Kaito, confident that he has memorized the questions and the answers, volunteered for the Battle Quiz. While Kaito got the first five questions right, after the fifth question Amanuma pressed the button in the middle without having him or Kaito read the questions, leading to Kaito swiftly lose. However, Kurama then told Amanuma the truth about Sensui, how he was only using him and how he would die if Sensui's plans succeeded. The reveal shook Amanuma and left him unsure of what to believe, allowing Yusuke's team to score fourth win after Kurama won the mini-game Three Sevens and defeat Amanuma. By the rules of the game the Gamemaster would die if beaten, and so after Kurama won Amanuma immediately died, leaving Kurama angry over the fact that he had to kill a boy to save the world and determined to get to Sensui.
However, Koenma later used his pacifier, or the Mafuuken, to resurrect Amanuma. This was also part of Sensui's plan, as the revival of Amanuma would weaken the power of the Mafuuken. After coming back to life, Amanuma went back to playing video games, but also developed a new worldview at started to make friends, attending a private junior high school.