Izaya Orihara (折原 臨也, Orihara Izaya) is a powerful and skilled underground informant. He usually gives information for his own enjoyment, and claims to love the entire human race except for Shizuo Heiwaijima. Izaya likes to start wars with him, just like in Episode 25, and then let him chase him around. At the end of the episode, Shizuo falls for the same trick as he did in Raira, and gets hit by a truck.

Izaya has the appearance and expression of one who is extremely crafty and confident, an appearance which he lives up to. Like his younger twin sisters, he enjoys causing trouble; he claims to love humanity, and greatly enjoys putting people in miserable or chaotic situations in order to observe their reactions. He also has a thorough understanding of human behavior and tendencies and is able to determine people's thoughts and predict their reactions with high accuracy. Izaya is cunning and charming, but he is not very interested in women and women are not interested in him. He has a very jolly (although this it can also be seen as mocking) personality, speaking informally with most people as if they are close friends and often using nicknames (such as "Shizu-chan" for Shizuo and "Dotachin" for Kadota Kyohei), and he even talks like a girl on occassion, especially online where he actually does pretend to be female.

Izaya Orihara likes to play a twisted board game which involves moving a combination of Chess, Go, and Shogi pieces on a Chinese Go board. He plays this game by himself in his room. The combination of games may be a representation of the many plots being acted out throughout Ikebukuro. Izaya Orihara's game may illustrate how Izaya watches over and manipulates the various plots being played in Ikebukuro. The rules he plays by are also beyond normal people, which seems to be a metaphor for the way he lives his life. He does not think his board game is a serious representation of the world however, as he usually prefers to observe people acting as they see fit instead of controlling them, and he even once set fire to his board game in his excitement.

Although Izaya enjoys conflict, often showing up wherever there is potential for one, he usually stays out of it, preferring to observe the fight instead of directly taking part in it. However, when provoked, he quickly puts his opponent in his place, simply proving his superiority without actually beating the other person. Therefore he is usually seen as the one starting fights, but hiding in the shadows instead of directly taking part. He uses a flickblade and is skilled in parkour.

Izaya is also known to be one of the strongest men in Ikebukuro, although the title of strongest man belongs to Shizuo Heiwajima. When Mikado Ryuugamine first came to Ikebukuro, Izaya was one of the people Masaomi Kida warned him to stay away from, simply describing Izaya as "dangerous."