Akuma

Akuma (in Japanese, "devil"), known in Japan as Gouki (豪鬼 Gōuki, ごうき?), is a video game character from the Street Fighter series of fighting games. Akuma made his debut in Super Street Fighter II Turbo as a secret character and hidden boss. In the Street Fighter series storyline canon, he is the brother of Ryu and Ken's sensei Gouken. Gouken and Akuma studied under a man named Gotetsu. Over time, Akuma became obsessed with pursuing forbidden energies in search of invulnerability. Once he found a way to harness the power of the Dark Hadou, he embarked on a wave of carnage that began with the killing of his own master.

At some point he apparently killed Gouken, which causes Ryu and Ken to want to defeat Akuma. However, in Street Fighter IV, Gouken is shown to be alive. Since they all trained in the same school of martial arts, Akuma's fighting style resembles Ken and Ryu's. Akuma fights only to test his and his opponent's strength. This means that he will only fight opponents that will provide a challenge, hence his refusal to fight unworthy challengers under any circumstances.