Shadow the Hedgehog (シャドウ・ザ・ヘッジホッグShadō za Hejjihoggu?) is the arch-rival of Sonic the Hedgehog. He was created as the "Ultimate Lifeform" by Professor Gerald Robotnik, using the DNA and blood of Black Doom. Shadow is immortal, and does not age. In contrast to Sonic, Shadow is typically portrayed as an antihero, or a renegade who can just as easily serve the forces of evil as those of good. According to an official poll, Shadow is the second most popular character in the series, second only to Sonic.
Shadow prefers to work alone and distances himself from essentially everyone, with few exceptions. He is soft-spoken and reserved, but he never bluffs; if he makes a threat, he has every intention of carrying it out. This is seen in Shadow the Hedgehog, when Shadow warns Sonic in one of the storylines that if he were to try to stop him, the black hedgehog would destroy his blue counterpart. Shadow is known to treat others in a very callous manner, throwing in his opinion no matter how cold-hearted it is and not caring for whatever the response is. In combat he displays a natural "killer instinct" that Sonic lacks. At times, Shadow is the most violent and merciless character in the series.
Alongside Shadow's dark demeanor, he can be smug, stubborn, and somewhat sarcastic. He is shown to be highly arrogant and self absorbed; in the 2006 video game, he (as Super Shadow) states that their battle against Solaris may actually be "a fair fight." Shadow constantly refers to himself as the Ultimate Lifeform and thus belittles characters as well as insulting their lack of strength. He shares a competitive ego that is close to Sonic's. He is also consistently serious, even when not on a mission, and he has no regard for silliness.
On occasion, Shadow appears to have some degree of mental instability. This can be seen when he would often have hallucinations about Maria's face, or through his numerous threatening speeches. He appears to improve with amnesia, but this tendency returns in the Shadow the Hedgehog game, where, in the various possibilities, he often pursues his foes with disturbing determination. The canon ending of Shadow the Hedgehog showed Shadow willing to destroy anything in his way to get to Black Doom.
Shadow possesses a strong sense of identity, as evidenced when after 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. Because of this, he possesses a strong will, making him rarely susceptible to being manipulated by other forces. This is especially evident with his encounters with Mephiles the Dark, where, even after learning of the possibility that he was sealed away for all of eternity due to humanity fearing his potential and feeling he was responsible for Iblis's ruin of the world, Shadow still refuses to aid Mephiles in his plans, and states that "he determines his own destiny."
Shadow is not without sentimental emotions and has shown a small sense of compassion, as seen in the Sonic Heroes opening scene for Team Dark. When Omega begins his attack, Shadow could have easily fled and left Rouge. Instead, he defends her against Omega, despite having absolutely no memory of her. Also, after Sonic has completed one of Shadow's missions in the DS version of Sonic Colors, Shadow admits the purpose of his mission was to ensure that Sonic was physically fit enough to continue his journey towards Doctor Eggman. Even with his lack of trust towards humans, Shadow would team up with them to fight for the greater good. He was also shown to be legitimately shocked and horrified when he learned that his gaining the Chaos Emeralds to restore his memories were actually manipulated by Black Doom in order to wipe out humanity. Likewise, when the GUN Commander finally confronts him, Shadow agrees to face judgement if he was indeed responsible for the ARK massacre 50 years ago.
It is possible that Shadow does not have much of a sense of humor or happiness, or he doesn't like others to see him acting in such a manner. Shadow brushes off any 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." Later in the game, Vector tried to cheer him up, but Shadow ignored it.
Despite Shadow being considered neutral in terms of alignment, he is considered an anti-hero by the most part with his overall motivation which is dedicating his life to protecting mankind. He has also been selfless, such as nearly giving his life at the end of Sonic Adventure 2 to save Earth. He seems to despise evil above everything else, especially the Black Arms. Regardless, Shadow is still neutral because he does whatever is necessary to get what he wants.
Shadow has a companionship with Rouge the Bat that is based mainly on partnership during the games. The two are teammates in Sonic Adventure 2 and Sonic Heroes, and often work together during mission stages, but they do not spend time in the other's presence outside of duty alliance. This marks them as occasional teammates when it comes down to appreciation, respect, and consideration over the term of "best friends." Shadow has not shared the closeness of opening up to neither Omega or Rouge the way best friends would do, let alone accompanying them as casual company due to his independent and distant nature, unlike how Sonic interacts with his companions. However, Shadow was implied to have at least heard of her before their formal meeting onboard the Space Colony ARK, as Shadow responds, "Now I know who you are! You're that government spy, Rouge the Bat, aren't you!" when stumbling upon her true intentions at the controls of the Eclipse Cannon.
Shadow has a very close relationship with Omega who, other than Maria, may be Shadow's closest friend. This relationship is first shown when Shadow looks out for Omega in Sonic Heroes halfway into the game. This was also shown in Shadow the Hedgehog when Shadow willingly helped Omega in Iron Jungle and Lava Shelter. In Sonic the Hedgehog, both Shadow and Rouge were somewhat saddened by Omega being reprogrammed to hunt down Shadow in the future, due to Shadow being seen as too strong of a lifeform, causing humanity to fear him. The bond between the two may have been strengthened by the fact that Shadow is the Ultimate Lifeform and that Omega is considered the Ultimate Robot and share a common importance to the world.
Shadow has had several altercations with the detective agency, the first being a confrontation in Sonic Heroes that escalated into a brawl, with their differences then seemingly set aside later when they joined forces against an evolved Metal Sonic. The Chaotix resumed their role as allies in Shadow the Hedgehog, with each member participating as a partner in the hero missions of various levels. Individually, Vector perceives Shadow as a friend, as demonstrated in one of the endings of Shadow the Hedgehog when the crocodile attempted to console him when he concluded himself to being nothing more than a living weapon of mass destruction, though Shadow apparently didn't want to hear it. However, Shadow seemed constantly annoyed with Vector as he repeatedly reminded him to find the computer room.
Espio has shown his capacity of friendship for the black hedgehog and the two proved to make a good team when they confronted Dr. Eggman. When Eggman tries to lie and manipulate Shadow, Espio tells the hedgehog not to fall for the doctor's tricks and to defeat him first. Charmy has exercised a more enthusiastic relationship with Shadow, demonstrating how much he respects and looks up to him while often stating how "cool" Shadow is. All the Chaotix voices are present in supporting Shadow during the last story as well as Expert Mode.
- Air Shoes: Allowing Shadow to travel along a path of rings at light speed.
- Mystic Melody: Play at the ancient shrines to open up new paths or shortcuts.
- Ancient Light: Allows Shadow to attack enemies in a row quickly.
- Flame Ring: Allows Shadow to somersault in a fiery fashion, allowing him to destroy metal containers.
- Chaos Spear (カオススピア): Shadow hurls two bolts of raw Chaos energy at an enemy. These lightning bolts, while armor piercing, may either hit a single enemy, or tail off and hit an adjacent enemy.
- Chaos Rift (カオスリフト): Shadow opens a rift in time and space, then launches it at a foe in an attempt to banish it. If the foe remains, the foe may become distracted
- Chaos Blast (カオスブラスト): Shadow concentrates his usual Chaos Blast into a sphere of energy within his hand, and delivers it in concentrated beam of chaos energy that may weaken affected targets.
- Atomic Strike (Atom Strike アトムストライク): Shadow and Sonic rapidly spinball around the opposing team, creating an electrical field around them and damaging everyone inside.
- Focus Field (Focus Shot フォーカスショット): Shadow hits a foe with a Chaos Blast as Omega simultaneously fires with his machine guns, in a dual attack which can leave the foe vulnerable.
- Metal Storm (メタルストーム): Shadow lifts Omega using Chaos Control, and Rouge tornado-kicks the robot into a foe, with a chance to stun them. Chaos Control allows Shadow to stop time, whether it be to attack in an instant or escape danger.
Shadow was ranked as the 2nd favorite Sonic character only beaten by Sonic the Hedgehog himself in the official Sonic character pool. In the Sonic Character Tournament held by the Sonic News Network, Shadow was voted the second most popular character by the fans. Out of the thousands of votes that were tallied, Sonic only won by ten. Game Revolution described his character as a "quintessential anti-Sonic." In a 1UP.com feature about "How Sega Can Save Sonic the Hedgehog," Jeremy Parish wrote that of the "tagalongs" that the series must ditch, Shadow's character was the most in need of it.
- Though the design of Shadow the Hedgehog was meant to be a secret until such time SEGA decided to reveal it in all its glory, details of the character were leaked when ReSaurus announced they were making figures of Shadow and Rouge, much as they had for the first Adventure title. Ironically, the company went out of business before they could release these figures to the masses.
- Shadow presumably has green blood, as Black Doom said Shadow was created from a sample of his blood, which is green.
- Prima's Official Game Guides for Sonic Adventure 2 called Shadow's Super form "Hyper Shadow" (despite how Sega and Sonic Team have referred to it as canonically being "Super Shadow"). Prima also said in their Sonic the Hedgehog (2006) guides that there are rumors that Rouge has feelings for both Shadow and Knuckles.
- In his Mario and Sonic at the Olympic Games artwork, Shadow appears to be missing his tail.
- Shadow has been proven to be very strong because in Shadow the Hedgehog, he is able to overturn heavy objects with ease whilst holding a weapon in the other hand. He is also able to destroy walls as well as carry some heavy objects without effort.
- In Sonic Riders and Zero Gravity, Shadow will reach high speeds without the use of his jet shoes after gaining the Attack Mode power-up.
- Shadow's skin color is usually tanner in his 2D artwork than his 3D artwork.