Izaya's Knife

Izaya is often seen using a folding knife similar to the, spyderco centofante 3, as his weapon of choice. Izaya is shown to have it hidden on his person and can deploy the knife swiftly before and during the midst of combat, showing his skill with knives. His folding knife is actually NOT a switchblade, since switchblades are knives that deploy the blade, utilizing a button on the handle to eject the blade out into the open-fixed position.

He is also shown throwing ordinary knives in the OVA.

History
Izaya is first shown using this weapon against the bullies harassing Anri, using it to cut and steal the purse of one of them. In almost all his confrontations with an offender, he is shown to use it as a means to defend himself.

In the anime Izaya was shown using a stiletto folding knife in high school to cut Shizuo.

Later, Izaya is seen in the Abadashi Arc, sporting a new pocket knife (similar to a Gerber Paraframe I) against Haruna Niekawa. Niekawa attempted to attack him while he was off guard for information about Takashi Nasujima. The knife is a silver folding knife with a paraframe handle and a curved drop point.

He appears to be using a folding kukri while fighting Shizuo in the last episode of Durarara x2 Ketsu as seen in episode 11.