Knuckles the Echidna

Knuckles the Echidna is a major character in the  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 A Super Mario World 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
Knuckles was previously confirmed to return in Super Smash Flash 2 as part of the initial roster for the game. However, following the release of v0.7 of the SSF2 Demo, the developers revamped the whole roster in order to accommodate some new, unexpected characters. Currently, Knuckles' fate as a playable character is unknown.

Trivia

 * Though he is not playable in the Super Smash Bros. series, he is one of eight characters in the  series to later have a costume for Mii Fighters based on him in and Super Smash Bros. Ultimate. He shares this distinction with Tails, Isaac, Chibi-Robo, Bomberman, Mega Man X, Zero, and Lloyd.
 * Knuckles also appears in Super Smash Bros. Ultimate as an Assist Trophy, making him one of seven characters in the SSF series to later appear as Assist Trophies in the official games. He shares this distinction with Shadow, Waluigi, Krystal, Isaac, Bomberman, and Zero.
 * For April Fools' Day 2019, McLeodGaming announced a supposed remake for the original Super Smash Flash titled Super Smash Flash 1 HD, which would have featured updated higher-quality sprites resembling those from Super Smash Flash 2. Knuckles was among those presented with an updated sprite (following a similar sprite style to and  in SSF2). Said mockup sprite was created specifically for the joke by former developer FriendAlias.
 * This mockup sprite would later be used in the fake trailer "Flash of Shadows", FriendAlias's proposal for McLeodGaming's April Fools' Day 2020 joke, in which Knuckles appears as a trophy.