Izaya's Knife

Izaya's knife is Izaya's weapon of choice, and he is frequently seen brandishing and using it in the anime. He keeps it hidden on his person and is shown to be a skillful wielder, able to deploy it in the midst of combat and even use it to parry Saika's thrusts. In almost all of his confrontations, he is shown to use it as a means of defense.

Dollars/Mika Harima Arc
Izaya is first shown using this weapon against the bullies harassing Anri Sonohara. He draws the uses the knife so quickly that Mikado Ryuugamine, who is watching, does not notice until after he has cut Youko's purse strap and shaved a part of Hiroshi's head.

In the anime, Izaya was shown using a stiletto folding knife in high school to cut Shizuo Heiwajima during their first encounter. In episode 25, he uses throwing knives against Shizuo.

Adabashi Arc
Izaya has a new knife in the second season that he is seen using against Haruna Niekawa when she attacks him for information about Takashi Nasujima. During his fight with Shizuo in the final episode, he is using a folding kukri.

Trivia

 * Izaya's knife in the first season resembles the real-world spyderco centofante 3.
 * Izaya's knife in the second season is a silver folding knife with a paraframe handle and a curved drop point that resembles a Gerber Paraframe I.
 * Since switchblades deploy the blade using a button on the handle that ejects the blade into an open and fixed position, it can be argued that his knife is not a switchblade.