"Knucklehead" (by Sonic), "Knuckie" (by Rouge),
"World's Greatest Treasure Hunter",
"Mr. Knuckles" (by Cream)
Team Sonic
martial arts, finding treasure, the Chaotix
competing with Sonic, fighting
messing with gadgets or tools,
being around women a lot, being tricked,
being embarrassed,
people messing with the Master Emerald
Sally Acorn, Bunnie Rabbot,
Antoine Depardieu/D'Coolette, Rotor Walrus,
Amy Rose, Shadow the Hedgehog, Rouge the Bat,
Cream the Rabbit, Charmy Bee, Espio the Chameleon,
Vector the Crocodile, Silver the Hedgehog,
Blaze the Cat, Colonel Robert Hogan,
Spin Dash
Swimming
Gliding
pretty much anything with grape
Come on, I’ll take all of you on! 
— Knuckles the Echidna
Knuckles the Echidna is the guardian of the Master Emerald on Angel Island and one of Sonic's best friends. In BearfootTruck's universe - particularly ASMW - he also serves part-time with the Knothole Freedom Fighters. Other than that, he's practically identical to his canon counterpart. For other universes' versions, go here.
Backstory
ASMW
Knuckles' backstory hasn't changed much here. He was born to unknown parents without any knowledge of why he was here, just that the he had to protect the Master Emerald. Then Robotnik came along and set off the events of Sonic the Hedgehog 3/Knuckles. At some point after this, Knuckles signed up as a part-time member of the Knothole Freedom Fighters. The reason he signed on part-time is because he still has to tend to the Master Emerald.
Sonic Journeys
Knuckles' backstory hasn't changed much here. He was born to unknown parents without any knowledge of why he was here, just that the he had to protect the Master Emerald. Then Robotnik came along and set off the events of Sonic Chaos. Since then, he has returned to guarding the Master Emerald, but will join up with Team Sonic on certain occasions.
Sonic War Zone
Knuckles' backstory hasn't changed much here. He was born to unknown parents without any knowledge of why he was here, just that the he had to protect the Master Emerald. Then Rouge the Bat came along and stole the Master Emerald for the Roseinians. However, Sonic and Miles "Tails" Prower help him to recover the Emerald. From that point on, he will occasionally join Sonic and Tails in the Great War.
Personality
Again, not much to speak of here. While Knuckles is still a hothead, he seems a bit more mellow here. Most prominently, he doesn't fight so often with Sonic. Sure, the two get into plenty of arguments, but compared to Shadow the Hedgehog, Knuckles is on very good terms with Sonic.
Abilities
As with other characters, Knuckles retains all the abilities he had in the games. Strangely enough, he hasn't used his digging ability in these stories, even though he could've done so in "Two Kinds of Heroes".
Appearance
Again, no changes here, but like Sonic & Tails, his appearance is the subject of much debate.
Items & weapons
So far, Knuckles hasn't been shown using any weapons or using vehicles. As far as he's concerned, his fists are good enough.
Fanfictions
ASMW
Two Kinds of Heroes
Knuckles is guarding Knothole when Sonic & Tails drag him along for a mission to destroy one of Robotnik's mech factories. However, he & the other Freedom Fighters are teleported to Stalag 13 during a freak electromagnetic storm. Knuckles is eager to beat up some German soldiers, but first, they have another matter to take care of: Colonel Robert Hogan and his team are planning on blowing up a German tank factory that’s supposed to build the new E-75 heavy tank.
During the plan, Knuckles stays behind with Sally Acorn, Bunnie, LeBeau, Antoine & Kinchloe and keeps watch for the guards while Sonic attacks the tank factory. After a minor confrontation with Shadow the Hedgehog, he assists him in destroying some Cylon Centurions. With that taken care of, he heads back to Mobius and helps to destroy the mech factory.
The Fakest Things Alive
Knuckles is fighting with Rouge over a Chaos Emerald when Blaze the Cat informs him that two of the Sol Emeralds have been stolen. He directs Blaze to Eggopolis while he heads over to Knothole. Here, he learns that Sonic has gone missing and that the future is doomed without him. Back in the present, he assists Silver in the search for Sonic. When Sally informs Knuckles that Sonic (an impostor, actually) has been found, he decides to go home, but Silver wants him to help rescue Blaze. Knuckles insists upon going home, but after some heavy-handed persuasion from Silver, he reluctantly agrees to go to Robotnik's lair with him.
With Blaze rescued and Silver back in the future, Knuckles goes back to Knothole, where the fake Sonic grabs his nose. After this, he returns to Angel Island to guard the Master Emerald. Up here, Knuckles meets Sonic again (actually another impostor) and witnesses him jump off the island. Thinking that Sonic has died, he returns to the Master Emerald, distraught. However, Rouge later informs him that Sonic's not really dead, which improves Knuckles' spirits. She also tries to steal the Master Emerald, but Knuckles gets into a tickle fight with her.
And Down Will Come Sonic – Cradle and All
Knuckles makes a very brief appearance in one of Sonic's dream sequences. He tells them that Robotnik once used a saw-equipped robot in an attempt to deforest Angel Island. However, Knuckles is actually the evil clone of Sonic in disguise.
Mighty Sonic
Knuckles, like most of Sonic's friends, is a fan of Super Changin' Muscle Rangers (a thinly-disguised version of Mighty Morphin' Power Rangers). His favorite character is Frankie Thompson, the green Muscle Ranger. He gets into a brief argument with Sonic over the show before heading back to Knothole. When Sonic informs everybody of the Muscle Rangers' kidnapping, Knuckles initially thinks that Sonic is giving away the plot of an upcoming episode. When it turns out to be for real, he accompanies Sonic to Robotnik's HQ. Piloting the RhinoVord, Knuckles helps to defeat Robotnik & rescue the Muscle Rangers. In the end, he sneers at Rouge for flirting with Oliver, causing her to think he’s jealous.
Other stories
Sonic Hotcakes
Knuckles gives Robotnik some hotcakes because he admires his guts. Unfortunately, Robotnik doesn’t admire the gift and sics his badniks on Knuckles. Fortunately, he gets away.
A Blue Light For the Dark
Knuckles appears in the "Sonic Says" segment for this story. He is walking down the streets of San Santos when he notices Mephiles soliciting donations for the Salvation Marine Corps. After some thought, he decides to donate $50.
Sonic's Weird Birthday
For Sonic's birthday, Knuckles gives him a literal "knuckle sandwich" just because he thinks the Blue Blur deserves it. He has a good laugh, but Sonic doesn't.
In this episode's "Sonic Says", Knuckles is attending a party in Knothole, but he isn’t feeling too good (presumably from eating or drinking too much), so Tails has to take him back to Angel Island.
A Dark Birthday
Knuckles has gotten a Chaos Emerald for Shadow the Hedgehog and a bottle of wine for Rouge for their birthdays. Then, Sonic comes along and informs him that Shadow already has a Chaos Emerald. Rad Red then gets angry because he suspects that the Ultimate Life Form is using him to give the Emerald to Rouge. Later in the story, Knuckles gets frustrated when Tails delivers a rather long-winded explanation about what he did to Shadow & Rouge's motocross bikes.