Akuma

The name "Sheng Long" comes from the Japanese "shouryuu", meaning "rising dragon". The kanji used in "shouryuu" was pronounced as if it was Cantonese. One of Ryu's win quotes in the English version of the Street Fighter II series of games said, "You must defeat Sheng Long to stand a chance". At the time, fans of Street Fighter thought that Sheng Long had referred to a person - the supposed master of Ryu and Ken, and not one of Ryu's moves (the Shouryuu Ken).
The creation of Akuma comes from an April Fool's joke in Electronic Gaming Monthly magazine where the editors claimed that the player can fight Sheng Long in the game by not touching M. Bison, the final boss of the Street Fighter II series. Sheng Long would then come out of nowhere and kill Bison, and the player would fight him. Sheng Long was supposed to have the moves of Ryu and Ken, but did much more damage and was also faster. To make this joke more believable, fake screenshots were made. Soon after, many fans, believing that the joke was true, flocked to Street Fighter II series machines, hoping that they could fight a nonexistent character. When asked about whether the joke was true, Capcom did not confirm nor deny it as the sudden infusion of money into their machines was driving up business; Capcom USA was taken in by the gag, and actually contact Capcom Japan and asked if the Sheng Long thing was legit. Only much later was this joke was revealed as such, by Electronic Gaming Monthly themselves.
Ironically, in Super Street Fighter II Turbo, parts of this story became true. Like the joke, Akuma would appear from out of nowhere and kill Bison, and the player would fight him, and similarly, Akuma had the moves of Ryu and Ken, and was much faster and dealt much more damage. As many people were accustomed to Sheng Long, many people called Akuma erroneously by the name Akuma Long. The move that was used to kill Bison (the Shun Goku Satsu) was also erroneously called the Bison Killer.
In the beginning, Akuma was exclusively a secret character and final boss, appearing in other Capcom games such as X-Men: Children of the Atom, which is considered the forerunner of the Marvel VS. Series.

Adon

Adon is a character from the Street Fighter series of video games.
According to his official biography, Adon was born in Thailand, with no year or date specified, but he 5-7 years younger than Sagat. He stands 182 cm and weighs 73 kg. He is skilled at wai khru, a Thai dance (a pre-fight ritual dance, similar to Krabi Krabong), and likes Muay Thai, his fighting style. He dislikes Sagat and people with big mouths.
Adon became Sagat's student three years after Sagat became God of Muay Thai. Frequently likened to a "lesser version of Sagat," even though he strived to differentiate himself with more acrobatic techniques, Adon entered his World Warrior tournament to prove he was more than that. The cocky Adon let his guard down when he faced Ryu, who took him out with a single Shoryuken.
Adon was not mad at Ryu for defeating him and Sagat, he was mad at Sagat for being unable to uphold Muay Thai's honour (conveniently forgetting how Ryu had pasted him much easier). He challenged Sagat for the title of God of Muay Thai and won, but the enraged Sagat had done so much damage to him in the fight that Adon had to be hospitalized for months.
When he was on his feet again, Adon heard that a mysterious fighter had killed one of his challengers, using the same power that Ryu had used to defeat Sagat. Adon swore to find the stranger and take his power for his own. To this day, it is believed that Adon still seeks him out.