There was surprisingly little information released about the kitsune in the series. In the episode The Girl Nextdoor we were introduced to Amy Pond, a youth friend of Sam Winchester who turns out to be a supernatural creature of the same species that his father and brother were chasing. The episode did show the viewers that the girl was able to shift her eyes to a yellow color with slits for pupils. On top of that, Amy was able to extend and retract her nails into claws. These claws are the main weapons of the kitsune on Supernatural and the thing that makes them most resemble werewolves on the show.
In this particular episode, there is a lot of cutting back and forth between the past when Sam discovers Amy’s true identity for the first time, and the present when he meets her again. The past Amy is faced with a hard-knock life in which she must consume human brains against her will in order to survive. While her mother has no issues with the diet and the violence that comes with it, the young girl struggles with the unethical side of it. When she tells Sam to escape before the return of her mother, it’s already too late. In a final attempt to protect her only friend, Pond kills her own mother by stabbing her through the heart.
Meeting again in the present, it turns out Amy has needed to resort to killing humans herself in order to save her ill child from dying. The kitsunes of Supernatural are after all dependent on human brains and cannot survive without them.
It is also addressed that originally, Amy was only using the brains of people that had passed away already, through her work in a mortuary. Had it not been for the illness of her son and her knowledge that only a fresh brain could save him, she would not have taken to killing at all. This is one of the examples of ethical monsters in the series, which Sam aims to protect while Dean’s approach is rather ‘better safe than sorry,’ advocating to kill all creatures regardless of their alignment. In the end, under the guise of going to fetch something, Dean kills Amy behind Sam’s back which is the last time a kitsune is seen alive in the show.
While the kitsune of Supernatural are first and foremost human with the ability to gain fox-like features, the original Japanese spirits are full-blood foxes with the ability to shift into a human form. The myths portray them as kind and friendly spirits with an occasional trickster nature. Like foxes, they are considered wise and do not have as much of a negative connotation as their Supernatural counterparts. As they grow older and wiser, the number of tails the kitsune has increases, up to nine for the wisest ones. Aside from companions they are described as messengers of Inari, the fox spirit.