An ill-tempered, super-strong young man with a bartender's outfit in Ikebukuro who carries intense animosity towards Izaya.
Shizuo Heiwajima roughly translates to "serene man peaceful island" in English, but despite the name Shizuo is actually quite high-strung and prone to violent outbursts. Even worse, he happens to have natural super strength, being able to rip signposts and vending machines out of the ground and throw them at whatever's pissing him off at will. In one fight scene, he actually punched a thug so hard that he flew out of his own clothes before smacking into the ground.
The in-story explanation of Shizuo's super strength is that unlike most people, he doesn't carry a mental block that tells people
Shizuo Age 7
"stop" whenever they are carrying something that is too heavy to be carried by normal muscles. As a result, Shizuo often found himself going to the hospital because of his habit of picking up any heavy object he could find and throwing it at the nearest offender. Even as a kid, he tried carrying a whole refrigerator or a desk by himself and succeeded, but the strain on his body kept sending him back to the hospital where his muscles and bones grew stronger as they regenerated. At some point, he could rip lampposts and newsstands out of the ground and use them with very little strain.
He was briefly roommates with Shinra, Celty and Izaya in high school, but Izaya kept pissing him off by secretly sending gangs of thugs to attack him to the point where Shizuo tried to hunt him down one day. However, Izaya kept escaping. Even today, Shizuo attacks Izaya whenever he sees him around, as Izaya later framed him for a crime he did not commit.
Shizuo's violent tendencies made it hard to hold down any job for more than a short length of time, even the bartender job he supposedly has the uniform for, so he ended up taking a job as muscle for an old friend named Tom, who happens to be a debt collector. Usually, the mention of Shizuo alone is enough to scare people into paying back their debts, but when they don't, Shizuo is not afraid to throw a giant object at them.
These days, he is still friends with Celty, often chatting with her for advice or to calm down. For some reason, she is the one person who does not cause Shizuo to get horribly violent with little provocation. Despite Shizuo's violent outbursts and fights, he has clearly stated that he himself hates violence and one day he wishes to have the strength to control his anger.