A kunai (苦無) is a Japanese dagger, possibly derived from the masonry trowel. The two widely-recognized variations of the kunai are short kunai (小苦無 shō-kunai) and the big kunai (大苦無 dai-kunai). Although a basic tool, in the hands of a martial arts expert, the kunai could be used as a multi-functional weapon. The kunai is commonly associated with the ninja, who used it to gouge holes in walls. Kunai were originally made to be farming tools but soon evolved into the weaponry they have become today.
Many ninja weapons were adapted from farming tools, not unlike those used by Shaolin monks in China. Since kunai were cheaply-produced farming tools of a useful size and weight and could be easily sharpened, they were readily available to be converted into simple weapons. As a weapon, the kunai is larger and heavier than a shuriken and with the grip could also be used in hand to hand combat more readily than a shuriken.
As with ninjutsu, the exaggeration persistent in ninja myths played a large role in creating the popular culture image of kunai. In fictional depictions, the kunai is commonly portrayed as a steel knife that is used for stabbing or particularly throwing, confusing it with the shuriken. Kunai were not designed to be used primarily as throwing weapons, though they can be thrown and cause damage.
In Shinobido[]
It's one of the weapons of the Mosu Ninja and their leaders Usuba and Ageha. Whenever they are out of melee range, the Mosu will throw kunai that, while unrealistically precise, are easily blocked.
Normal Mosu troops can throw up to three at a time, but Usuba and Ageha can sometimes throw groups of four or more, and combined with their ability to duplicate will quickly overwhelm the player if not stopped.
Nagi is shown to use kunai for surprise attacks during cutscenes, but for some reason she doesn't use them in actual combat.