There was one element of the Sonic the Hedgehog 2 trailers that kept longtime fans and newcomers on pins and needles waiting for the release date. That element was not the return of the Blue Blur, the new unhinged Eggman, or the first film's post-credit reveal of Tails, it was Knuckles the Echidna.
Introduced in 1991's Sonic the Hedgehog 3, Knuckles the Echidna is the third or fourth most important character in the Sonic canon. He's been a fan favorite since his initial release, and Idris Elba's casting to the first on-screen appearance of the character made him a massive point of excitement.
The changes made to Knuckles from game to film are deceptively simple. Fans who listen to him talk might be easily convinced that he's closer in tone to Dragon Ball Z's Vegeta than Sonic's musclebound red ally. His writing is distractingly similar to that of characters like Teen Titans' Starfire or Guardians of the Galaxy's Drax the Destroyer. His dialogue is weighty, he talks constantly about honor and pride, and he has strong opinions about the way that people should behave. When Eggman mistreats his minion Stone, Knuckles sees it as dishonorable. When Sonic saves him from drowning, he is both shocked by his behavior and immediately indebted to him. He comes off as a cruel villain from the beginning, but he softens over time. Some of his traits might come off as deviations from his in-game personality, but they are less like changes in character and more like changes in tone.
The Knuckles of the game franchise is hard-headed, slow-witted, and muscle-bound. He's driven by duty. Tasked with protecting the Master Emerald, he spends a ton of his time standing guard over its location and awaiting any danger. Whenever a threat appears, real or made up, he doggedly pursues his new enemy and fights until he is either victorious or convinced otherwise. Like in the film, his first appearance in the game saw him tricked and manipulated by Eggman until Sonic sets him straight and puts him on the path of good. He's intensely good-natured, but easily led astray. He defines himself by his fighting skill and his desire to do the right thing. He joins Sonic in his battles against Eggman and every other threat that arises with the same fun-loving ambition that lights the Blue Blur. Knuckles, as he appears in the film, is a version of the character free from the elements that the later games introduced, but given more screen-time than ever before.
Though his dialogue is changed, his personality is fascinatingly similar. He's still a bit slow, but part of that comes from his fish-out-of-water routine on Earth. While both he and Sonic are relatively new to the planet, Sonic's pop culture knowledge is encyclopedic. Many of Knuckles' funniest lines come from his lack of understanding of his new environment. His fighting spirit comes from his warrior culture. His slow wit comes from his general unfamiliarity. He's always been a confident braggart; his lines are just a bit more erudite now. It's a perfect updating of the character from the earliest moments of the franchise. The later games introduced a laid-back attitude and a strange surfer-like method of speaking. Knuckles always had personality, but his new backstory better fits the tone Idris Elba brought to the character.
In both iterations, Knuckles comes from a long line of Echidna, of whom he is the last. The Echidna have a strange history that bends depending on the work they're being featured in. A lot of their lore is established in Sonic Adventure, which told the story of the clan's war against rival Echidna clans. Knuckles' people committed several acts of violence that awakened an ancient evil and unleashed destruction onto the world. The film's version sees Knuckles' father lead the violent elimination of owls, like Sonic's caretaker Longclaw. There's clearly a lot going on that hasn't been revealed in the backstory of that rivalry. Perhaps Knuckles' spin-off series is set to explore that, but the effect on the character is similar. He's not the deepest character, but he never has been.
Knuckles may come across as a very common character in stories like these. He's the powerhouse who tries his best, he doesn't quite understand what's going on, but he means well and people love him for it. He isn't a classic character turned into a common archetype — rather, the version of him that never got a voice just happened to fit well into that mold. The film universe version of Knuckles has swiftly become one of the fanbase's favorite characters, for good reasons. Without making too many changes, Knuckles just becomes a more modern take on the classic character, and that's an impressive translation.