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. He is also fluent in Russian, which is exhibited in the final episode and episode 13 when he speaks with Simon. Izaya has two younger sisters, Mairu and Kururi, who are twins and troublemakers. He claims to have grown up in a normal family whose only abnormal trait was their naming habits. He does not like his sisters and finds them to be unlovable, saying that if even he can not like them, he expects no one else to ever be able to either. His name comes from a combination of the biblical Isaiah and Japanese for "the one who looks over the crowd." In the novels, Shinra is the only person that Izaya doesn't consider as one of his "humans." Rather, Izaya considers Shinra as his only friend (which is rather rare).

An arrogant, selfish, and enigmatic Archer-class Servant. He appears familiar with Fuyuki City and claims to have a past relationship with Saber. Typically clad in an ornate golden armor, he possesses an immense number of Noble Phantasms, though none are representative of his true identity. Gilgamesh is the great half-god, half-human king born from the union between the King of Uruk, Lugalbanda, and goddess Rimat-Ninsun. He ruled the Sumerian city-state of Uruk, the capital city of ancient Mesopotamia in the B.C. era. He was an ultimate, transcendent being so divine as to be two thirds god and one third human, and no others in the world could match him. He was a despot possessing high divinity who believed he was invincible. He is not merely a legend, and is said to have actually existed and ruled during the Sumer Dynasty five thousand years ago. He was the King of Heroes who possessed all things in the world, whose tale is recorded in mankind's oldest epic poem, the Epic of Gilgamesh which portrays Gilgamesh as a hero, destined to be king and achieve great feats, who is driven to meet his destiny, facing challenges together with his best friend Enkidu. His title, King of Heroes, is not meant to call him a king who is a hero, but instead implies that he is the king over all heroes. He is mankind's oldest hero, the origin of all myths and model on which heroes are based, so his story is copied within the mythologies of all the countries of the world. The heroes of various myths are derived from his legend, so his Gate of Babylon possesses all of their Noble Phantasms. Though there are several heroes holding the title of "King," the King of Knights and King of Conquerors, he is the only one in all of heaven and earth crowned with the title of "King of All Heroes." In Fate/kaleid liner PRISMA☆ILLYA, he's the Heroic Spirit sealed into the eighth Class Card. He manifests as his younger self due to Illya separating him from his shadow form half-way through his incarnation. While initially an adversary, he quickly becomes an ally to fulfill his objectives. Gilgamesh in his child form has a more easygoing attitude compared to his adult self. He asks Illya to call him Gil-kun.