The Kunai is a throwing blade and the first Ghost Weapon Jin unlocks. It staggers nearby enemies on a quick throw, giving you an opening to finish them or escape a crowd.
Equip it with R2 and left on the d-pad, then aim and throw with R1. It shines against a single tough enemy: stagger him, then follow up with a heavy attack or a finishing blow.
Hidden Blade (1 point) adds a blade to each throw. Serrated Blades (2 points) doubles the damage each blade deals. Both sit under Ghost in the Techniques menu.
Ghost Weapons are tied to Legend. Each of your first rank-ups opens a new tool, and the Kunai comes first.