Shadow debuted in the game Sonic Adventure 2 as one of three playable "dark" characters, alongside Rouge the Bat (who also debuted in this game), and Doctor Eggman. Dr. Eggman set Shadow free from a G.U.N. base on Prison Island, where he had spent approximately 50 years in suspended animation. In exchange, Shadow agreed to aid the doctor in his plans to take over the world; holding to himself his true intentions as wishing to get revenge for the death of his friend, Maria Robotnik, as he thought she wanted; it is strongly implied that his belief that Maria's final wish was revenge against humanity stemmed from Gerald Robotnik modifying Shadow's memories between Maria's death and Shadow's encapsulation. To achieve this, Shadow revealed the Eclipse Cannon to Dr. Eggman, who later destroyed half of the moon in a demonstration to the populace. Beforehand, he also stole the green Chaos Emerald from a bank, unknowingly also framing Sonic the Hedgehog for the deed.
He later participated in the second heist alongside Dr. Eggman and Rouge the Bat against Prison Island, where he would plant explosives throughout the island as part of their getaway, although his resemblance to Sonic also resulted in Amy Rose mistaking him for the latter. However, because Tails came to Amy's rescue from Dr. Eggman, he was forced to act fast due to the unexpected delay. In addition, after Rouge ended up getting caught and stranded in the containment area while recovering the four Chaos Emeralds, Shadow, largely because he realized that his plans of avenging Maria Robotnik would be for naught if Rouge perished in the explosion, rushed to get to Rouge and by extension the Chaos Emeralds.
After meeting Sonic for a second time, Shadow also believed that Sonic was trying to copy him. They fought to a stalemate before Shadow was forced to leave to rescue Rouge (with Sonic also learning from Eggman via Shadow's radio that the island was about to blow up), and just barely arrived at Rouge's location and used Chaos Control to warp himself, Rouge, the four Chaos Emeralds and presumably Eggman out at the last possible second. After the demonstration, needing the final Chaos Emerald to fulfill their plans, Shadow pursued Tails' plane at a canyon. Although he was unable to stop them, he nonetheless reported to Eggman that they are headed his way. When Eggman discovered the presence of two apparent emerald readings, and thus deduced Tails' plan of placing a fake Chaos Emerald into the cannon, Shadow departed, apparently to confront Sonic and Tails to stop them for interfering, although Eggman stopped him, wanting to deal with them personally. After Sonic was seemingly killed by Dr. Eggman via a rigged escape capsule, Shadow, witnessing the explosion, guessed that Sonic was only an ordinary hedgehog.
Afterwards, Shadow stumbled upon Rouge's plan to steal the Chaos Emeralds, and deduced that she was in fact a spy for the President of the United Federations, although Rouge then unveiled a report from Gerald Robotnik regarding the Biolizard, claiming, it, not Shadow, was the actual Shadow. However, Shadow was forced to depart after Eggman mentioned an anonymous figure (Sonic) heading towards the Eclipse Cannon, although not before stating that he's still Shadow despite his memories possibly being fake, and that "the fake emerald is good enough for Rouge." He then discovers Sonic the Hedgehog at the area. Shadow, impressed at Sonic's capability of using Chaos Control despite using a fake Chaos Emerald, nonetheless stated that he cannot allow him to live before fighting on a collapsing bridgeway. In the last story, Eggman inserted the final Chaos Emerald, only to initiate a destabilization program on the ARK. Professor Gerald Robotnik then revealed that he had made this function to have revenge for Maria, and that the ARK would destroy the Earth. Sonic, Eggman, Rouge, Tails and Knuckles took a shortcut to the cannon's core so they could stop the Professor's mad plan.
Amy Rose then helped Shadow realize Maria's true wish. In a twist of fate, Shadow ultimately aided Sonic when realizing, with Amy's help, Maria's true wish, which was to save humanity. Meanwhile, Sonic the Hedgehog and Knuckles had reached the central core, where they reactivated the Biolizard. Notably, a scene from Gerald Robotnik's diary appeared again. After Shadow defeated the Biolizard, Knuckles used the Master Emerald to stop the Chaos Emeralds. But the Biolizard, being the prototype of Project Shadow, used Chaos Control to fuse with the ARK, continuing its fall. Then, Sonic and Shadow, in their Super States, destroyed the Biolizard. The Ultimate Lifeform prototype became a monster, codenamed Finalhazard. Using their pooled energies to destroy the earth, but Shadow and Sonic defeated him and executed an enhanced Chaos Control which stopped the fall, and warped the ARK. Afterward, Shadow fell to the Earth and was presumed dead.
-Shadow the Hedgehog
In Shadow the Hedgehog, Shadow is revealed to be the dual creation of the evil alien overlord Black Doom and Professor Gerald Robotnik. Gerald could not finish the project on his own, so he contacted the Black Comet and used Black Doom's blood and his DNA to create Shadow. Shadow eventually gathers all of the Chaos Emeralds, which Black Doom takes and uses to warp the Black Comet to Earth. Shadow regains the Chaos Emeralds and transforms into Super Shadow to confront Devil Doom, Black Doom's powered-up form. It is worth noting that, if the battle timer passes seven minutes, Eggman reveals to Shadow that he sent one of his robots to save him after the incident in Sonic Adventure 2, and that he is the actual Shadow and not a Shadow Android. After the fight, Shadow destroys the Black Comet with the Eclipse Cannon. In the end, Shadow puts his past behind him and chooses to move forward.
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