Shadow is a brooding loner, to put it bluntly. He usually acts with a cool and businesslike indifference, only occasionally showing his vulnerable side. He is rarely seen with others for an extended period of time and usually distances himself from essentially everyone. He is usually blunt and doesn't speak much, but he always knows exactly what to say in combat. However, Shadow never bluffs. If he makes a threat, he has every intention of carrying it out.

Shadow also does not mind boasting about how powerful he is, calling himself the "Ultimate Lifeform" and, like Sonic, can be extremely arrogant, often belittling his opponents and insulting their lack of strength. He believes no one can beat him and tends to see anyone as a fool for challenging him. 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 lead to fear and 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.

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 is and not caring for whatever the response is. Shadow is also shown to have a sharp wit, like in Sonic Adventure 2, where after Sonic calls him a faker, he replies that Sonic isn't worth being considered his fake, and in Shadow the Hedgehog, he taunts Black Doom's eye for having trouble with GUN. This attitude is reflected in the original Japanese version, where Shadow omits honorifics and speaks highly of himself (if not rudely), though contrasting this, in Japanese he normally uses the "boku" pronoun when referring to himself as opposed to the more arrogant/confident "ore" pronoun (which characters such as Sonic use).

On occasion, Shadow appears to have some degree of mental instability, in particular post traumatic stress disorder. 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 the instances of the latter game were simply an effect of his amnesia. On one occasion, in Sonic Adventure 2, seeing police sirens flashing caused him to intensely flash back to the massacre aboard ARK and in particular Maria's death. 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 Sonic Forces, it is heavily implied that Shadow ruthlessly slaughtered an entire mercenary band not only in an extremely dismissive, nonchalant manner (not even remembering the incident a few months later), but also seemed slightly proud, mocking the mercenary defense squad by saying they themselves needed a defense squad. He also demonstrated rather coarse and even profane language in his own game.

Shadow also possesses a strong 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.

Despite his apathetic nature, Shadow is not without compassion. 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. In Sonic Colors DS, Shadow was also shown to be worried about the possibility of Sonic and Tails getting hurt while trying to save the wisps, much to their surprise. However, he remained dismissive about his concern for them. He spent the majority of Sonic Chronicles: The Dark Brotherhood trying to find Omega out of worry for his well-being. He was also quite enraged when he found out that the Marauders had been hurting Omega and helped him to get revenge.

Shadow is often depressed about the loss of Maria, which is why he keeps to himself, but his relationship with Maria is obvious as he goes to great lengths to protect the planet and keep her dream of peace and prosperity alive. He also maintains a strong bond with his teammates, Rouge and Omega, proving that while his attitude makes him seem like a careless person, he does have some compassion. Shadow brushes off most kindness or sympathy that is sent his way; in Sonic Heroes, when Rouge showed concern for Shadow when he saw a broken android of himself, suspecting Team Dark's Shadow might be an android too, Shadow curtly brushed her concern away. In Shadow the Hedgehog, after Sonic suggested some "friendly competition," Shadow bluntly rejected his offer. At times, however, Shadow is shown to appreciate genuine care. In Sonic Adventure 2, after Amy made him remember his promise to Maria, Shadow openly decided to keep his promise to protect the planet both to Maria and her. Another instance is in Sonic the Hedgehog (2006), when Rouge told him that she would always be by his side, he acknowledged her.

Despite the fact that Shadow often fights for the greater good, he is considered an anti-hero for the most part due to his morality and means to his desired end goals. 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.[13] Alongside Rouge he has also taken prize money and jewels upon winning in Sonic Free Riders. He and Rouge disregard the opening and closing ceremonies for the Grand Prix, showing up late without a third teammate and, if they win the Grand Prix, starting to walk off right away with what turn out to be dummy versions of the prize money and gems. Regardless, Shadow does have his moments of selflessness and heroism. He nearly sacrificed 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 him, in Sonic the Hedgehog (2006).

Ref: https://www.youtube.com/watch?v=zUD08Rz-rWQ