Shizuo is a smoker despite being seen wearing his usual trademark bartender suit and blue sunglasses. His hair is blond, although as a child it was brown. Most of the time his personality is rather simple; he is normally quiet and nondescript, and usually simply follows Tom around as his bodyguard. However, Shizuo has a very short fuse, and when angered, his body "acts on its own," and he fights with incredible strength. Despite his destructive tendencies, he apparently doesn't like violence. He is, along with Izaya Orihara and the Dollars, one of the people Masaomi Kida warned Mikado Ryūgamine to stay away from, telling him to 'not hesitate to run from him' in the manga.
Normally, the brain restricts one's power, shutting down muscles in response to pain in order to prevent serious injury; unfortunately (or fortunately, depending on how you look at it), Shizuo doesn't have this ability, and is capable of using his full strength at will. His power is described in the novels as the naturally unlimited form of human strength, and Shinra Kishitani likens it to the incredible "adrenaline power" that people get in times of emergency. Although he was always injured as a child, the fact that he could always fully exert himself meant that his body quickly strengthened itself and caught up. Now, as an adult, he is able to use his power without destroying his own body.
Despite the fact that Shizuo uses his power very often, he doesn't like violence, and apparently hates the fact that he is so strong. This is probably the result of events in his childhood where his strength caused others to fear him, alienating him from others at school, and even at times inadvertently hurting people he cares for. It wasn't until his run-in with the Saika Army that he was able to accept himself the way he was, since they claimed to "love" him because of his strength. Shizuo then let himself loose; outsiders believe that the "Slasher" incidents ended when he went after Shizuo (it was actually when Anri defeated Haruna Niekawa, but Shizuo still beat up many of Saika's daughter blades and only stopped fighting when they did).
Shizuo is also capable of often superhuman parkour moves, most likely a result of his own strength and from years of hunting down Izaya, who is very good at running away from him. Both of them share an intense animosity, and simply seeing him on the street usually results in Shizuo hurling a trash can/vending machine at him which he flees while loudly yelling "Izaya" at him.