Fog Imposters (also referred to simply as imposters and clones) are enemies that appear in the Story Mode of the Nintendo 3DS version of Mario & Sonic at the London 2012 Olympic Games, used by Bowser and Dr. Eggman to attempt to help them prevent the Olympic games from happening. They are versions of the character formed from the Phantasmal Fog that are able to imitate the real characters, though are less skilled in Olympic Events.
Fog Imposters appear throughout the Story Mode, where they appear in various episodes as opponents in Olympic Events. Fog Imposters first appear in The Night Before, where a pair of blue Mario and Sonic imposters appear and scare Toad and Omochao, who initially mistake them for the actual characters. In Stadium Blues, a pair of blue Sonic and Tails imposters appear and challenge Mario and Luigi, who defeat them, causing them to disappear and the Phantasmal Fog to thin. After this, Omochao arrives and explains that they were imposters, and that the ones that he and Toad had seen last night had gone on to scare other people, and after encountering Dry Bones and Dry Bowser in Foggy Foes, who are protecting the Fog machine, hypothesise that Bowser has something to do with them. A pair of pink Peach and Blaze imposters appear and challenge Daisy and Amy Rose at the beginning of Morning in Hyde Park, and after being defeated disappear, only for another six to appear and chase Daisy and Amy, eventually causing them to collapse. Six more Peach and Blaze imposters appear at the start of Peach in a Pinch attacking Peach, Blaze and the blue, yellow and purple Toads, and are once again defeated and disappear. After finding out that Rouge has been seen around, they believe that she may be responsible for the Fog Imposters, though later find out when her memory has recovered in The Mysterious Thief that they are the work of Dr. Eggman. Sonic and Tails encounter red Mario and Luigi imposters in Sonic Arrives, and come to the same conclusions as Mario, Luigi, Toad and Omochao when they defeat them and find King Boo and Boo protecting a fog machine in The Ghostly Tower. Sonic and Tails along with Yoshi encounter a pair of orange Shadow and Silver imposters in The Blocked Bridge, who are once again defeated and disappear. Sonic and Tails explain the situation to Yoshi, and are overheard by Shadow and Silver who explain that they have also defeated some more imposters. After finding Jet in the area, they incorrectly believe that he may have something to do with it, though also later find out from Bowser Jr. that it is instead Dr. Eggman. Mario, Luigi, Toad, Sonic and Tails meet up in Mario and Sonic, where they find that they have all found the same information, before defeating a purple imposter each of the four Heroes characters. In A Rainbow In Fog, another four Heroes fog imposters are defeated, although this time the imposters are stronger due to the more powerful rainbow-colored fog. Another four rainbow Heroes imposters are then defeated in The Promise to Toad. Upon initially seeing Bowser and Dr. Eggman when powered up by the Phantasmal Fog in The Grand Showdown, Tails mistakes them for Fog Imposters. Dr. Eggman creates and defeats a single grey Sonic fog imposter using the Fog urn in The Fog Returns, where it is explained that they are created when the holder of the urn thinks of someone, originally as a curse to deter looters of the ruins it was found in.
Fog Imposters take the form of several different characters, though lack some more specific details, most notably irises. They appear in various shades of the color of fog that they are formed from, and will change through multiple different colors when formed from the more powerful rainbow-colored fog. Imposters are silent and unable to talk in any way, though some do have speech bubbles which consist entirely of ellipses.
The Fog Imposters take the form of whoever the person holding the fog urn is thinking of at the time, which was created as a curse to deter looters of the ruins in which the urn was located. Fog Imposters appear to have the ability to appear from the fog and disappear back into it at will, though will disappear if they are defeated in an Olympic event. When Fog Imposters disappear in this way, the fog around them will thin, though the fog is always thicker around the imposters. While Fog Imposters mainly seem to have a somewhat physical form, as they are able to make contact with the sports equipment and other athletes in various events, they also appear to be able to become intangible, as a group of them were able to pass though the gates of the British Museum.