Knuckles the Echidna

Knuckles the Echidna is a major character in the Sonic series as well as the rival and friend of Sonic the Hedgehog; he is the guardian of the Master Emerald. He makes an appearance in Super Smash Flash as a playable character.

Character description
During the development of Sonic the Hedgehog 3, the development team wanted to create a character that would become Sonic's rival due to the fact that Sonic had been lacking a rival up until then. The development then came up with Knuckles whose conception is based off several designs presented to be Sonic's new rival.

In the Sonic series, Knuckles was born on Angel Island and is the last surviving member of the Knuckles clan, an ancient clan that was nearly wiped out by Perfect Chaos over 4,000 years ago. Knuckles then went on to protect the Master Emerald located on Angel Island.

In A Super Mario World
Knuckles briefly appears in the second episode of the A Super Mario World series along with Sonic, both serving as Bowser's guardians. Sonic and Knuckles' reason for working for Bowser is that they wanted to prove SEGA characters are better than Nintendo characters (which is ironic as Bowser is a Nintendo character) by challenging Mario and Link in a fight. Sonic briefly escapes the battle after Knuckles is defeated by being pushed into the lava. He is not seen afterwards.

In Super Smash Flash
Knuckles is a starter character from the large Sonic series roster in Super Smash Flash. He has five attacks, all of which deal high damage, especially his rock throw which can do 180% and one-hit KO opponents.

In Super Smash Flash 2
Along with Shadow the Hedgehog, Knuckles' status as a playable character in Super Smash Flash 2 is currently unknown.

Trivia

 * Though he is not playable in the Super Smash Bros. series, he is one of five Super Smash Flash characters to later have a costume for Mii Fighters based on him in Super Smash Bros. for Nintendo 3DS and Wii U. He shares this distinction with Mega Man X, Zero, Tails, and Lloyd.
 * Knuckles was intended to be a playable character in Project M, and was very close to being ready for release before the project ceased development. Though this is not counted as an official case of a Super Smash Flash character later becoming officially playable in the main Super Smash Bros. series, as it is with Sonic, Cloud, Meta Knight, or Mega Man, it is still worthy of note for fans of the project. Isaac shares this distinction, but was much more incomplete than him at the time of PM ' s cancellation.
 * In Super Smash Bros. Ultimate, Knuckles is a non-playable character and he is one of the Assist Trophies along with Shadow and Bomberman, two of the playable characters in the SSF series. He also appears as a background character on Green Hill Zone in the Super Smash Bros. series.