Koekai (口径, Kōkei) is a katana, wielded by Killua Arakaki. It is an ancient sword, being hundreds of years old.
Koekai is stained with the blood of its victims. It is impossible to wipe away the blood, as it has soaked into the Obsidian and dried, making it part of the blade. It is filled with ancient Japanese markings on it. Obsidian was the main material used during the time this sword was crafted.
Killua's knowledge about weapons lets him use Koekai to its full potential. He can easily shatter weaker blades, deflect attacks from large blades, and even fend off against multiple opponents at once. The katana is extremely light, flexible, and durable.
- The sword was missing for over 400 years until Koekai found it.