Fifty years later, Shadow was released by Dr. Eggman to help him conquer the world. Initially, Shadow sought to destroy Earth to avenge Maria, who had been killed by the military, but was persuaded to help save it from Gerald's doomsday plans. In the process, Shadow lost his memories and he sought to uncover his past, during which he met the Black Arms. Ultimately, Shadow thwarted Black Doom and saved the world. From there, Shadow continued his mission to protect humanity by becoming a special agent of GUN.
Shadow is sharp-witted, always on edge and has a strong sense of purpose, willing to accomplish his goals by any means. Essentially a dark version of Sonic, Shadow can match him in both speed and skill, and can use Chaos Control with a Chaos Emerald. While preferring to work alone, he has helped Sonic and his allies when needed.
Role in the Series
Sonja and Shadow used to be enemies at first until Shadow decides to join with Sonja and her friends at the ending of Sonja's Adventures of Shadow the Hedgehog. Shadow refused at first in Sonja's Adventures of Sonic Adventure 2 but he is good friends with Hannah/Evil Sonja.
Shadow is a brooding loner, to put it bluntly. He is rarely seen with others for an extended period of time, and distances himself from essentially everyone. He is usually soft-spoken, silent and only says what he needs to before a fight. However, Shadow never bluffs. If he makes a threat, he has every intention of carrying it out.
Shadow constantly refers to himself as the Ultimate Lifeform and like Sonic, he can be extremely arrogant, often belittling his opponents and insulting their lack of strength. Shadow also has a ruthless and merciless edge in combat that all other characters in the series lack, and displays a natural "killer instinct".
Shadow's infamous ruthlessness and aggression leads to fear and a nervousness that inhibits characters from his unshakable intent or sheer power in combat. Numerous characters from the Sonic series have shown absolute dread from encountering Shadow. Shadow's enemies never treat him lightly; he is one of the very few people Sonic takes seriously and acts hostile towards.
Alongside his dark and violent demeanor, Shadow can be smug and stubborn. He treats others in a very callous manner, throwing in his opinion no matter how cold-hearted it might be, and not caring for whatever the response is.
On occasion, Shadow appears to have some degree of mental instability. This is demonstrated by his instances of post-traumatic flashbacks to Maria's face prior to and during the events of Shadow the Hedgehog, though it could be argued that they were simply an effect of his amnesia. Another example could be that in many games Shadow is featured in, he pursues his foes with disturbing determination; that is to say he'll go to any length to accomplish something he desires or feels should be a certain way, no matter what the cost. In his own game, he also showed a coarse language.
Shadow also possesses some sense of identity, as evidenced when after falsely being told that he isn't the Ultimate Lifeform due to a lab report. He states to Rouge that even if his memories were fake, he is still Shadow the Hedgehog. Similarly in Sonic Heroes, during the Egg Fleet level, he mentions that even without his memories, he is still the Ultimate Lifeform, Shadow the Hedgehog. These qualities make him rarely susceptible to being manipulated by other forces.
Shadow is not without sentimental emotions and has shown a small sense of compassion from time to time. When Omega begins his attack in Sonic Heroes, Shadow could have easily forgotten about Rouge and engaged Omega without defending her. Instead, Shadow saves her from Omega, despite having absolutely no memory of her. He was also shown to be legitimately shocked when he learned that his goal of gaining the Chaos Emeralds to restore his memories was actually manipulated by Black Doom in order to wipe out humanity. Likewise, when the GUN Commander finally confronts him with his own memories of Shadow's nightmarish creation, Shadow agrees to face judgment if he was indeed responsible for the ARK massacre 50 years ago.
Shadow brushes off most kindness or sympathy that is sent his way. This is shown in Sonic Heroes, when Rouge showed concern for Shadow when he saw a broken android, Shadow curtly brushed her concern away. Also in Shadow the Hedgehog after Sonic suggested some "friendly competition," Shadow turned his back to Sonic and said, "Don't waste my time".
Despite the fact that Shadow often fights for the greater good, he is considered an anti-hero by the most part due to his gray morality. He does whatever is necessary to get what he wants or feels is right, though this enthusiasm causes him to take risks and jump into situations without fully thinking it through. Regardless, Shadow does have his moments of selflessness, helpfulness and heroism. He nearly gave his life at the end of Sonic Adventure 2 to save Earth and helped Silver uncover the truth of the Solaris Project, after saving Sonic from Silver.