Shikigami

Shikigami (式神) are a kind of spirit summoned to serve a practitioner of onmyodo, similar to a western witch's familiar. Shikigami cannot be seen by most people. Although invisible, shikigami could take a variety of human or animal forms, possess or bewitch people, or even cause harm or bodily death.

In InuYasha, Shikigami came in the form of Tsubaki's Hebi (snake/serpent), Kikyo's Shinidamachu, and Kocho and Asuka, the two girls who accompany Kikyo. There were also others, but those three come to mind as the most obvious examples.

In InuYasha: Secret of the Cursed Mask, this is the main power of Kaname Kururugi and Michiru Kururugi.