Shadow the Hedgehog

After being conceived by Takashi Iizuka, the character of Shadow was first shown to the public in a game trailer, however his name was not revealed. GameSpot described what little was shown as an "evil version of Sonic himself - similar in appearance, but with darker skin, more angled eyes, and a fearsome snarl instead of Sonic's trademark grin." After his popularity grew to being right below Sonic, Yuji Naka felt that they could use his different identity from Sonic to build upon the Sonic brand and thus focused on him for a new game.

Shadow's debut was in the game Sonic Adventure 2, where it is explained that he was created fifty years prior to the game's events, by Professor Gerald Robotnik, who had been charged with the task of researching a path to immortality. He created Shadow as "The Ultimate Life Form", a first step towards achieving that goal, but the project was shut down and his granddaughter, Maria, was killed in the aftermath. Before she died, she ejected him towards Earth in an escape pod, but Gerald, driven insane by her death, designed a program that would cause the research station, Space Colony ARK, to crash into Earth once all seven Chaos Emeralds were placed inside it. Back on Earth, he modified Shadow's mind so that he would carry out Gerald's plan if he was ever released.

Fifty years later, Shadow was released from suspended animation by Dr. Eggman, grandson of Gerald Robotnik, who sought after Shadow after learning from Gerald's diary that he was the ultimate life form his grandfather had created, hoping to use him to conquer the world. Shadow, under Gerald's programming, tricked Eggman into helping him collect all seven Chaos Emeralds with the help of Rouge the Bat, and in placing them in the ARK, almost allowed it to crash into Earth. Amy Rose managed to trigger his memory of Maria, and the ARK was placed back into orbit using his skill, Chaos Control, using up the last of his energy and him falling to Earth, seemingly dead.

In Sonic Heroes, Rouge discovered Shadow in a stasis tube in Eggman's secret base. Suffering from amnesia, Shadow joined forces with Rouge and Omega in order to track down Eggman; after defeating the final Eggman robot, Rouge stumbled into a room full of what appeared to be Shadow clones. Deducing that the Shadow she was with was also a clone, she then went on to fight Metal Sonic with the others.

In Shadow the Hedgehog, Shadow, still with no memory of his past, meets the Black Arms and Black Doom, who seems to know Shadow. Throughout the game, Shadow has constant flashbacks of his past. Black Doom also baits Shadow with the knowledge of his past, while Eggman tells him that he is simply an android created by Eggman to mimic the real Shadow. In the final story, Black Doom reveals to him that while working on his immortality project, Gerald Robotnik contacted Black Doom and made a deal with him; if Black Doom helped him with his research, Gerald would deliver the seven chaos emeralds to him in fifty years' time. Using his own blood, Black Doom helped Gerald create Shadow, an immortal life form that would both help with Gerald's research, and who would collect the chaos emeralds for Black Doom in the future. However, once Gerald learned of Black Doom's plot to take over the world, he created the Eclipse Cannon as a means to destroy the Black Comet, and left it up to Shadow to stop the Black Arms. While fighting Black Doom, Shadow also learned from Eggman that he in fact was not an android, but the original Shadow, whom Eggman had rescued with one of his robots after Shadow fell from space (Sonic Adventure 2). He destroys both Black Doom and the Black Arms, and decides to carry on living as he sees fit.

In Sonic Battle, Rouge saves Shadow while they are investigating Dr. Eggman's secret base. Believing that he is a weapon, he insists the people of Earth must destroy him, but it is revealed that he ultimately has the heart and soul of Maria Robotnik and so is not the enemy of humanity. In one of Gerald Robotnik's journal entries, he mentions that "Project Shadow" was put on hold because of Project Gizoid.

In the 2006 Sonic the Hedgehog video game, Shadow works with Rouge and Omega after confronting Mephiles the Dark. One of Mephiles' goals is to interfere with Shadow's future, but along with Silver and Sonic, he super transforms and manages to defeat Solaris for good, and Mephiles, as part of Solaris, with it.

Shadow appears in Sonic Rivals as a playable character alongside Sonic, Knuckles, Silver, and Metal Sonic, where he locates and uncovers the truth about Dr. Eggman. He also appears in the sequel,Sonic Rivals 2. He is an unlockable character in the game Sonic Riders, and is a Speed type character, whilst he also appears in the sequel, Sonic Riders: Zero Gravity. Shadow also appeared in the game Sonic and the Secret Rings, being the first unlockable character for the Party Mode and also appears in Super Smash Bros Brawl as an assist trophy and a sticker.

Future appearances include a role as a playable character in Sonic Chronicles: The Dark Brotherhood.

Shadow appears in Super Smash Bros. Brawl as a Trophy and an Assist Trophy. The player must unlock Sonic to get Shadow as one of the Assist Trophies.

Shadow has been confirmed to appear in Sonic Unleashed.

Shadow has been confirmed to be playable in Sonic and the Black Knight. He is an alternative to Sonic's single sword gameplay and Knuckles has dual blade action.

According to official profiles, Shadow was created long ago by Prof. Gerald Robotnik to be the Ultimate Life Form. Sharp witted and seemingly always on the edge, Shadow is described as the dark incarnation of Sonic the Hedgehog. Once he has set himself to a goal, he will do whatever it takes to accomplish it, regardless of any danger. After the trauma of the death of his first closest friend, Maria Robotnik, Shadow strives to fulfill his purpose and keep the promise he made to her, despite his general loathing of mankind. Although his relationship with Sonic seems to have developed from antagonistic to friendly rivalry, animosity from not understanding their different mindsets continues.

Shadow is ageless and immortal. However, Shadow shares a lot in common with Sonic. Although Sonic's speed is described as "second to none," Shadow can skate at speeds fast enough to rival the speeds the world's fastest hedgehog can run. He can also perform spin attacks common to Sonic, which are a variations on the tendency for hedgehogs to roll into tight balls for protection. Additionally, with the power of a Chaos Emerald, Shadow can warp time and space with Chaos Control. In contrast, the Archie Comics version of Shadow can directly access the "Chaos Force" and does not require a Chaos Emerald. Shadow is also able to use a variety of other Chaos powers, such as "Chaos Spear" and "Chaos Blast".

By harnessing the power of the seven Chaos Emeralds, Shadow is able to use super transformation to transform into Super Shadow. He is bestowed with the new abilities of flight and near invulnerability, like Super Sonic, and his normal abilities of speed and his Chaos powers are greatly enhanced.