Shizuo is a often seen wearing his trademark bartender suit and blue sunglasses. As a child he had brown hair, but dyes his hair blond. He is introduced to the viewer in the first few episodes as a violent, no-nonsense tough guy, on equal footing with seemingly dangerous characters such as Izaya. However, it is implied that most of the time, he is quiet and nondescript, simply following Tom around as his bodyguard. Shizuo apparently doesn't really like violence and has misgivings about his own strength; unfortunately, he has a very short fuse and when angered, his body "acts on its own" and he fights with incredible strength. Masaomi warns Mikado to stay away from him, along with Izaya and the Dollars, telling him to 'not hesitate to run from him' in the manga.
Shizuo's great strength is explained this way: 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 limitation, and is capable of using his full strength. His power is described in the novels as the naturally unlimited form of human strength, and Shinra likens it to the incredible "adrenaline power" that people rarely display in times of emergency. Although he was chronically injuring himself as a child, over time 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 used his power very often, he was (and to some extent remains) worried about it. This is probably the result of events in his childhood where his strength caused others to fear him, alienated 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, giving him a reason to stop holding back. 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 gained from years of chasing Izaya, who is very good at running away from him. They share an intense mutual animosity, and Shizuo simply seeing Izaya on the street usually results in Shizuo hurling a trash can, vending machine or other dangerous projectile at him, followed by Izaya fleeing and Shizuo giving chase while angrily yelling "I-za-ya!" after him.
"I hate violence..."