Optimus Prime serves as the leader of the Autobots and transforms from a Peterbilt truck.
Ironhide reappears in the sequel as the weapons specialist and best friend of Optimus Prime.
Peace-loving Hatchet, the medic for the Autobots, also returns, taking the form of a Hummer H2.
Main character Sam Witwicky's friend and main form of transportation, Bumblebee, returns in his regular form, a Chevrolet Camaro, albeit for the second film he takes on the guise of a 2010 Camaro model.
Megatron appears in the second film, although director Bay denied the robot would make a reappearance.
Second in command to Megatron, Starscream, an F-22 Raptor Jet, works in conjunction with Megatron to capture Sam Witwicky.
Scorponok returns in mechanical scorpion form as one of the only robots unable to transform.
The Fallen serves as the leader of the Decepticons. He is characterized as the ultimate "bad guy," and is the original cause for the conflict between the Autobots and Decepticons.
Arcee, a female robot who transforms from a pink motorcycle and main mode of transportation for the love interest of Sam Witwicky, Mikaela Banes, finally makes an appearance in the series.
Soundwave, a Decepticon communications specialist in the form of a satellite orbiting Earth, almost did not make it into the sequel due to concern over how to execute his transformation.
Ravage, also in orbit, and another Decepticon who cannot transform, is launched by Soundwave down to Earth to steal an Allspark shard.
Jetfire, a geriatric robot, takes his form from an SR71 Blackbird, a Cold War-era reconnaissance plane. Originally a Decepticon, Jetfire joined with the Autobots after growing tired of the Decepticons' violent and destructive nature.
The Doctor, a spider-like robot, is a scientist and medic devoted to using his skills for torture and destruction. He attempts to remove Sam Witwicky's brain during the film.
Martial arts expert Sideswipe appears in the Transformers film. Originally conceived as a candy-apple red Lamborghini, he takes his form from a silver Corvette Stingray.
Jolt, the Chevrolet Volt plug-in hybrid, plays an Autobot in the movie, unlike his toy version, which is a Decepticon.
Wheelie or Wheeljack, while mentioned in the first film, finally makes an appearance in the second as a radio-controlled truck.
Twin brothers Skid and Mudflap, also known as "the Twins," take the form of a Chevrolet Beat and Trax concept car, respectively. The two play young, restless and dumb archetypal characters, although Mudflap attempts to appear more sophisticated and mature than his brother.
Sideways, a minor character, appears in the Shanghai battle.
The Devastator is formed by a combination of the Constructicon robots: Scavenger, Scrapper, Hightower, Longhaul, Rampage, Overload and Mixmaster. The Constructicons are a subfaction of the Decepticons.
More brawn than brains, the Demolisher, also a Constructicon, takes the form of a Terex Excavator bike and protects a number of Decepticons after the Shanghai battle.