Kitsune achieve and collect their Tails as they age, and grow in power and wisdom.
Tails are manifested as physical objects. Due to their enhanced coordination, Kitsune prefer to have the objects representing their Tails altered into weapons they can wield.
Tails are formed when a Kitsune triggers, uses or masters each of their supernatural talents. The more Tails a Kitsune possesses indicates the Kitsune's power level. The older the Tail is, the stronger the weapon shall be.
Kitsune share a synthetic relationship with their forged Tails. Like how the full moon is to werewolves, Tails are also the source of a Kitsune's power.
Noshiko used her Tails in her combat against the Nogitsune. She sacrificed her Tails to call forth the Oni from the Nemeton, and command the demons. Eight of her Tails were sacrificed to manifest the demons. The older the kaiken sacrificed, the stronger the respective Oni became. Noshiko's ninth Tail, her first and oldest, was stolen from her by the Nogitsune in De-Void. The Dark kitsune used the kaiken to enhance his FoxFire to perform mind control over the town's local werewolves Isaac Lahey, the twins and Derek Hale. In Insatiable, the Nogitsune sacrificed Noshiko's final Tail. He carried this out with the Oni Noshiko had previously summoned so close to him. This resulted in the control of the Oni being exchanged from Noshiko to him.
Because all of Noshiko's Tails have been sacrificed, her power is severely depleted so that she's now only "technically" a kitsune.
In Strange Frequencies, Ken slams and embeds Kira's shuriken into his desk. This action snaps Kira out of a Kitsune homicidal episode.