Chihiro

Chihiro 千尋 is a kitsune, a shape-shifting fox (all foxes are kitsune). 

They're much older that Hikari, but very young for a kitsune, and they are very childish and playful - although they occasionally show remarkable wisdom from experience. They don't really know humans that well, their family has always kept them away from humans (they live in an area where yokai are seen as dangerous creatures that should be eradicated), but they find humans fascinating, and so they ran away from their home to go live among them.

Chihiro has very little knowledge or understanding of human customs and morality, so Hikari often has to babysit them, get them out of trouble, and explain to them that "no, luring someone to a river and drowning him because he made a mean comment about your kimono is not a rational response." They grew into a sort of sibling relationship.

Worldbuilding info about kitsune in Hakatsuki no Monogatari:

Kitsune are agender, as far as human genders are concerned - they might have gender roles in fox society, but nothing that can be compared to human ones. Chihiro turns into men and women indiscriminately depending on the situation (this applies only to the version of the kitsune in my universe, not to Japanese folklore).

The older a kitsune is, the more powerful they become. They grow a new tail every hundred years (Chihiro is relatively young). Young kitsune don't control their shape-shifting very well, and can only hold a transformation a few hours at a time. If they're not concentrating enough, small aspects of their true nature will show - tail, ears, yellow eyes...

Even when perfectly transformed, their human form usually has fox-like features, hence Chihiro's very thin eyes, long pointy face and sharp teeth.

Kitsune also have other powers, such as creating fox lights (sorts of little fireballs) and illusions, turning into objects, and even possessing humans and draining their life force. Chihiro can make fox fires and very basic illusions.

Chihiro has a different name in fox language, which I haven't determined. As a human, they chose Chihiro because it sounded nice.