"People who call others stupid are usually more stupid. But I'm stupid, so I'll say it anyways...YOU'RE STUPID!"
Shizuo Heiwajima, also knows as 'Ikebukuro's Fortissimo', 'The Bartender of Ikebukuro' and 'Oh holy fuck there is a vending machine flying at my face' is, above all else, a peaceful man who is nondescript in his personality at most times. Anywhere else in the world, where reputation did not preceed him, he wouldn't even be glanced at twice. He spends most of his time following his employer, hands tucked into his pockets, calm as can be. And, contrary to what most might tell you, he's not always raging. Most of the time he's placid and unconcerned by the rest of the world- they can do whatever they like, really, it doesn't bother him.
However, when he does slip into a rage, it is spectacular.
Most of the time, Shizuo's anger is triggered by someone talking about or threatening someone that he considers a friend, such as Celty, Tom, and, of course, his brother Kasuka. He can also be triggered by a face he hates- namely, one Izaya Orihara. When he gets angry enough, he loses control over his temper, and his body, and he flies into action, his inhuman strength allowing him to lift heavy items such as vending machines or rip stop signs, lamp posts, and street lights straight out of the ground. Up until this point, he's been able to control himself just enough not to hurt any innocent bystanders.
He really hates violence.
He hates the part of himself that gets angry so easily, and when he feels anger building up steadily instead of just exploding all at once, he tries to calm himself, though usually this doesn't go so well. The few times he has managed it, it's been because the person he's been angry at didn't look capable of taking his destructive rage, and his fear of accidentally hurting them overwhelmed his instinct to lash out.
When around people he doesn't know, who have not as yet pissed him off, Shizuo is unfailingly polite, as he was raised to be, and he likes to be around people who aren't afraid of him... unless that person is, of course, Izaya Orihara, in which case the point is moot because just seeing his face angers the blond. He's loyal to those he considers his friends, even when it's possible that they're in the wrong, because in his mind, that's how friendship works- you stick up for them, even when they're not right.
In the end, Shizuo is a just a fairly normal young man with some anger issues.