Hunter Bracelet

Yuki recieved her bracelet from Kaien Cross, who performs the ceremony to link Yuki and Zero Kiryu without their knowledge.

The hunter bracelet is a piece of jewellery linked to a tattoo given to an ex-human vampire in order to tame them. The hunters perform a ceremony using the blood of the ex-vampire onto the bracelet. The bracelet holder then only has to touch the bracelet to the tattoo in order to disable the vampire.

It is unknown whether taming only works on ex-human vampires. It is not confirmed as to whether the bracelet will work for anyone wearing it, or only the individual who partook in the ceremony, it is most likely the latter as Yuki is known as the one who has tamed Zero.

There are probably other types of jewellery that can be used in the ceremony, but only Yuki's bracelet has been seen in the manga.