Shadow the Hedgehog (シャドウ・ザ・ヘッジホッグ, Shadō Za Hejjihoggu?) is an anti hero and the arch rival of Sonic the Hedgehog. Created as "The Ultimate Life Form" by Professor Gerald Robotnik, Shadow is indestructable, however that doesn't mean that he does not feel pain, and that doesn't mean that he cannot be killed. Shadow, being the ultimate lifeform, was created in a condition that causes him to never age. Since his debut, Shadow has appeared in a multitude of games, television shows, and comic books. His past was almost a complete mystery until the game Shadow the Hedgehog revealed the events surrounding his creation. In contrast to Sonic, Shadow is typically portrayed as a tortured anti-hero, or a renegade who can just as easily serve the forces of evil as those of good. Though lately, he has done absolutely no evil, only good, making him a hero. However, he does what he needs to do to accomplish what he knows to be right. So overall, Shadow could be considered a die-hard hero. According to an official poll, Shadow is the second most popular character in the series, second to none but Sonic himself.[1]

Amy Rose (エミー・ローズ, Emī Rōzu?), once known as Rosy the Rascal, is a character who appears in the Sonic the Hedgehog series. She is a pink, female hedgehog who plays the part of Sonic's self-proclaimed girlfriend. She has chased Sonic since her first debut in 1993, and has been trying to win his heart in everything from the series' animated TV shows to comics and games. She is the first female character introduced into the game universe and is also one of the most popular, coming in fifth place on an official popularity poll.[3] Like Sonic and other characters in the series, Amy is significantly shorter and lighter than humans her same age. To be precise, she is 12 years old, 90 cm (2'11") tall (although at times it looks as if she's almost the same height as Sonic), and keeps her weight a secret.[4] She was described as being 8 years old in Sonic CD, but as of Sonic Adventure and onwards she has grown older (by 4 years). Also, with the addition of Cream the Rabbit, Amy seems to have found herself a sidekick similar to Sonic's own Tails (as implied by Sonic Battle, Sonic Heroes, Sonic Advance 3, and Shadow the Hedgehog.) The alias "Rosy the Rascal" is most likely a play on of "Rosie the Riveter", a feminist character and national icon which represents the women that had to work during World War II when all of their husbands were drafted. The name is rather suiting as both characters are extremely tough and motivated females. Amy Rose is also noted to be very popular among fans of Sonic and also to remain attractive to the series' older members.

Knuckles the Echidna (ナックルズ・ザ・エキドゥナ, Nakkurusu za Ekiduna?), or "Knux" for short, is a red 16-year-old anthropomorphic echidna, who is both physically powerful and highly resilient. He has a white crescent birthmark on his chest. The character is both a friend and a rival to Sonic the Hedgehog. Knuckles' name is derived from the four knuckles, two on each hand, which are actually sharp spikes. According to official Sonic Team polls, Knuckles is the fourth most popular character in the series. His first game appearance was in Sonic the Hedgehog 3, released on February 2, 1994 for the Sega Mega Drive/Genesis. He was described as being 15 years old when he made his debut, but since Sonic Adventure, he is now given the age of 16. Knuckles has two spikes or claws on each hand, and his dominant trait is his strength. He can also glide by trapping air under his dreadlocks, and is very skilled at a wide variety of martial art forms.