Sonic - The star of the show, as much as Sonic loves to run, he also loves to relax. He is less than willing to be on the go during his off-time, much less help Eggman, whom he doesn't trust at all. Nevertheless he shows of some of his most awesome moves during the film, and of course saves the day with a heroic heart. Go Sonic! :)
Tails - Tails is still the adorable child prodigy, using his knowledge of aviation and technology to aide Sonic. He has an adorable fear of lightning. :D
Eggman - Eggman has his most hilarious role ever! Bouncy and devious, he makes a plan to copy Sonic's life data and use it to program his newest creation, Hyper Metal Sonic. While Metal Sonic is out doing his bidding, he makes his own personal plans... to marry the president's daughter, Seira. I would've never imagined Eggman wanting to get married... XD
The President - He's... the president. Residing on South Island with his daughter Seira, he is held hostage by Eggman for the first half of the movie and makes a pathetic attempt to rescue his daughter in the second.
Seira - The other new character, Seira's main concerns are her own wants and her father's well being... but mostly the former. She has a thing for Sonic and despite what some movie synopses say, she does NOT have a thing for Hyper Metal Sonic. Her name is sometimes spelled Sara or Sera, but I get the spelling "Seira" from the videogame she plays with Eggman - her name is on the screen. :)
Knuckles - Sonic and Tails find Knuckles in the inner dimension of the planet hunting for treasure. He rescues them from Black Eggman and joins them on their mission.
Hyper Metal Sonic - Eggman's newest creation is a metal version of Sonic containing all of Sonic's data, meaning he can mimic all of Sonic's moves... a powerful opponent. However, he has one flaw; he has everything about Sonic programmed into him, even a good side...