(As a preface: for the record, whenever I call Shadow 'emo', say he would shop at Hot Topic, call him an MCR fan, etc etc etc, please know that I'm saying it as an ex-emo teen, arguably-currently-emo adult. Which is to say, it's not only affectionate, but also a compliment. I love this edgy hedgehog.)
Shadow the Hedgehog is an artificially-created being, and the last member of the Black Arms species (as a hybrid made with their DNA). Technically, he was created 50 years ago, but he is mentally 15. His birthday is March 14th.
His creator is Dr. Gerald Robotnik, who is also the grandfather of both Maria Robotnik and Dr. Eggman. Gerald was requested to help with a medical project ("Project Shadow") meant to unlock immortality by the then-President of the United Federation. Gerald refused initially, but upon realizing that this could help to cure the terminally ill Maria, he agreed. He began to work on trying to create an immortal creature, dubbed "the ultimate lifeform". He also created Black Doom, the leader of the alien species, the Black Arms, and made a deal with him: in exchange for Black Doom's help and DNA, Gerald would have his creation deliver the Chaos Emeralds to Black Doom in 50 years. After an initial failed attempt which proved to be beastly and unruly, Gerald began to work on Shadow, this time giving him a heart and soul similar to Maria's, in hopes that this would ensure that he would not become a weapon of mass destruction.
Upon his creation, he was befriended by Maria, who he quickly grew very close with. Maria was a young human girl who was born onto the Space Colony ARK, and, due to her terminal illness being mitigated by the low gravity, was unable to leave. Shadow views Maria as his sister, and the two of them both dreamed of visiting the Earth together someday. However, their shared peaceful existence ended when G.U.N., a military group, raided the ARK to eliminate everything and everyone related to the project, due to believing it was too dangerous. Maria and Shadow tried to escape the raid together, and were able to get as far as the escape pods. With the G.U.N. soldiers right behind them, Maria initiated the escape pod's launch with Shadow inside, and was mortally wounded by gunshot afterwards. With her last bits of strength, Maria asked Shadow to protect the world, for her sake, which acts as a huge driving force for Shadow's character; though that memory being tampered with made him instead seek out revenge in her name at first. In a 1UP interview, Shadow said that if he could change one thing about his past, it would be to give up his life for that of Maria's.
Though Shadow's likes are oftentimes listed as 'nothing' (okay, emo kid), he actually is known to like a great multitude of things. Aside from the obvious (that being Maria), he likes multiple foods, like pizza, Doritos, snickers, Hot Pockets, chocolate and coffee beans (yes, just the beans, he eats them straight like a little freak). He also likes action movies (including Terminator as a named example), fighting games, Burnout 3: Takedown and Kill la Kill. Aside from disliking things that most people dislike (being insulted, being tricked, etc), he also dislikes wasting time and losing. He's also notably VERY bad with computers, which is really, really funny to me.
Personality-wise, Shadow is a broody loner. He's blunt and doesn't speak much, but he's aggressive, stubborn, and can be quite smug and callous. He also possesses a strong sense of identity. That said, Shadow also canonically struggles mentally; particularly with post-traumatic stress disorder, due to Maria's fate. He has been shown to have post-traumatic flashbacks, which can be triggered by high-stress events, or by flashing lights; seeing the police's sirens caused a notably intense flashback. That said, he works hard to heal himself of his trauma.
I have met multiple people who have said that Shadow was their first exposure to a character with post-traumatic stress disorder, and I can also confirm the same for myself. Generally speaking, Shadow was also my first introduction to the antihero archetype of character (see this page's title), and was a character who I really loved in my childhood. All of these things make him a very important character to me and my upbringing! Although I know that I would still love his character if I had been introduced to the series later. He's just such a delightful, cool and interesting little guy!!
Per usual, with all characters near and dear to my heart, I have given Shadow the Hedgehog the highest honor I can bestow: a headcanon that he's queer and autistic. Shadow being autistic is a decently common headcanon (see: blunt, emotional dysregulation, samefoods, generally keeping to himself, a common animation of brushing his spines back which can be seen as a stim... I could continue but I'll stop there), and he's also one of those characters that I can see being trans in any direction. I personally headcanon him as librafluid (which means mostly agender, but has a small amount of attachment to (a) fluctuating gender(s)), but there's no trans headcanon that I dislike for him. I also see him as arospec and MLM. Ask for a specific label and he'd say he's a faggot because he's 15 years old and looks Like That, and what do you expect?
Also re: Shadow being queer, Ian Flynn (the head writer for the official comic series and for the mainline game Sonic Frontiers) went on a podcast to extensively talk about Sonic and Shadow as a romantic couple, confirming that there is a foundation for it to work, so basically I win forever and ever.
ANYWAY ☆ FAST FACTS: CANON SHADOW THE HEDGEHOG EDITION! ☆