A young man who sells information for his own enjoyment. He claims to love the entire human race except for Shizuo Heiwajima, and exhibits said love by putting humans in often miserable conditions and observing their reactions.
He attended Raira Academy with Shizuo, Shinra and Kadota. According to Shinra, Izaya and Shizuo hated each other with a passion almost from the point they met. In the novels Izaya framed Shizuo for crimes on several occasions. In the anime he puts Shizuo in constant problems, such as gangs attacking him daily up to framing him of crimes.
He is a master of parkour, and also quite skilled with his flickblade (the blade he uses must be slightly opened before the mechanics of it springs opens the blade - a switchblade will open on its own after the user presses a switch) as he is implied to be the only one capable of leaving numerous cuts in Shizuo's bartender suit after a fight in the novels. In the anime, he shaves a gang member's head bald without him noticing it.
Izaya has two younger sisters, Mairu and Kururi, who are twins and troublemakers.
His name comes from a combination of the biblical Isaiah and Japanese for "the one who looks over the crowd".
an extended description!