Spoilers for season two of the anime and volume 8 and onwards of the manga. I will do my best to hide all the relevant bits. I am currently at chapter 123 of the manga, so please show me the same courtesy.
Garou is one of the best-written shounen villains I've ever seen. His appearance marks a step upward for the entire franchise.
A brief overview: Garou is a human with significant psychopathic leanings. As a child, he watched (too) many superhero shows and was always left with the same question: why don't the villains ever win? They work so hard and they are so powerful that it always seems like the heroes have to risk everything to get a chance to defeat them. So why does fate always ordain that they lucky, plucky heroes always come out on top? The luck of the heroes is unreal and decidedly unfair. By all rights, the villains should be victorious in many situations. This so affected Garou that he felt driven to make things the way that they should be...by becoming a villain so powerful that he could crush any hero.
He struggled in formal training under the world-class martial hero Bang. While he did learn much about technique, he became restless and violent in the structured environment - especially chafing because of Bang's allegiance to the Hero Association. One day, he snapped. After brutally beating down the rest of the students, he disappeared into the criminal underworld.
When the Hero Association faces a crisis of unprecedented proportions, they gather muscle from the underworld in the hopes of convincing them to help preserve humanity. Garou uses the meeting as an opportunity to begin his rampage, beating down both heroes and villains in order to establish his dominance. After leaving the meeting, he begins to hunt down heroes in earnest.
One after another, he begins brutalizing do-gooders. While determined, he doesn't rush ahead or throw away his life on a single fight. He's smart and strategic, using the heroes' own promotional material to learn about their abilities before fights and retreating from deadly situations when necessary. He's not a coward, however. When surrounded, he digs deep into a well of focus and willpower usually only reserved for shounen protagonists, displaying a combination of common sense and fighting skill that is both awesome and completely down-to-earth.
He's not the toughest guy - but he doesn't need to be. If he's getting hit, he's not fighting correctly. Instead of relying on the showy gimmicks and absurd powers of many other characters in the series, he watches his opponents in order to predict their movement, learn from them, manipulate them and break the flow of combat in his favor. This is especially important because of the inevitable fight between him and Saitama. Saitama is powerful, of course...but he's sloppy. As has been made the point of multiple Garou fights at this point, Garou has incredibly keen senses, can read body language and can evade, parry and counter like there's no tomorrow. It's not an unstoppable force versus an immovable object, it's an unstoppable force versus an untouchable object.
Garou's character is complicated. He has a majorly fucked-up outlook on things, but he's not a one-dimensional psycho. There's a lot of instances where it's shown that he's just very misguided, though that now has lethal consequences.
Saitama stans will be BTFO by the second season of the anime. Garou is just better and this will result in antifeats for Saitama.
His visual design is unbelievably good. OPM in general has an amazing mix of designs and I definitely appreciate another realistically-achievable lean build to balance out characters like Puri Puri Prisoner and Superalloy Darkshine. I could conceivably cosplay Garou, but only the most jacked of bodybuilders have any hope of accurately cosplaying Prisoner or Darkshine.
His name is officially translated as "Garou", but I once saw his name Westernized as "Garrow" and thought that that sounded exceedingly bad-ass.
He might straight-up be my favorite character in Onepunch-Man now.