Short answers, pulled from the story.
Akuma made his debut in Super Street Fighter II Turbo, the fifth arcade iteration of the Street Fighter II games, as a secret hidden character and alternative final boss. Players had to meet specific undisclosed conditions to trigger his appearance before the final match with M. Bison.
Akuma was designed by Akiman at the request of Noritaka Funamizu, based on direction from Yoshiki Okamoto referencing the Super Saiyan transformation from the Dragon Ball franchise. Akiman completed the entire design in a single hour. He modeled the character's face on a Buddha statue or a Kongo Rikishi statue, and the kanji "ten," meaning "Heaven," was added to Akuma's back by staff on impulse.
Akuma was designed partly to satisfy fans who had been misled by journalist claims about a hidden character named Sheng Long. Noritaka Funamizu wanted a character built off Ryu's design who would contrast sharply with the existing villain M. Bison. His hidden status in the game came from later development discussions aimed at making him feel "mysterious and really powerful."
Akuma is the younger brother of Gouken, who is the master of both Ryu and Ken. After defeating Gouken, Akuma takes an intense interest in Ryu because he senses in Ryu a power similar to his own, known as the Satsui no Hadou. Critics and writers have framed Akuma as a version of what Ryu could become if that darkness fully consumed him.
Akuma was chosen for inclusion in Tekken 7 by Namco staff member Katsuhiro Harada, who felt the character would fit naturally in the Tekken world. Because Capcom provided no character model, Namco built Akuma's entire Tekken presence from scratch. In the story mode, Akuma seeks to repay a debt to Kazumi Mishima by killing her husband Heihachi and son Kazuya, and his fight with Kazuya was left with an intentionally ambiguous outcome.
Akuma has historically been banned from all competitive tournaments of Super Street Fighter II Turbo, including updated versions, because even his weakened playable form is considered the most powerful character in the game. Critics described his moves as excessively difficult for both newcomers and experienced players, and reviewers from GamePro and GamesRadar singled out his unfair difficulty as a persistent issue.