Shikigami

are a kind of spirit summoned to serve a practitioner of onmyōdō. Some are used as charms for good fortune, some are used as amulets for protection, and some are used as curses. Shikigami usually remain invisible or disguised to human eyes, although those with spiritual powers can normally perceive them. Their power is connected to the spiritual force of their master, and they can take a variety of human or animal forms, possess or bewitch people, and even cause bodily harm or death.

Throughout the Series
In InuYasha, the shikigami came in the form of Tsubaki's serpent and Kikyō's two girl-shaped attendants, Kochō and Asuka.

They were also used by Momiji and Botan to send small paper-sized men towards Inuyasha and his crew. When they returned with a hair strand from Inuyasha and Kagome, the priestesses were able to transforms two of their shikigami into larger versions of Inuyasha and Kagome. These shikigami were able to somewhat replicate Inuyasha and Kagome's powers. However, the transformed Shikigami were unable to replicate the powers of items they were carrying.

In Yashahime, Konton have the shikigami for the magic with his servants Wind Lion and Thunder Lion, even to create the a giant Blue Ogre that ended up devouring his head.

Trivia

 * In The Secret of the Cursed Mask, invoking shikigami is the main power of Kaname Kururugi and Michiru Kururugi. However, it is utilized in a much different fashion than portrayed in the manga or anime, in that shikigami is described in a manner similar to Ki energy, being "a spiritual power that exists in all creatures in the universe" according to Kakuju while other characters, primarily Inuyasha and Shippo, describe Kururugi's use of it as a "strange light". Though with shikigami and onmyoudo being such an undefined power in Japanese mythology to the point it essentially lies between spiritual and magical power, seeing shikigami/onmyoudo take form in different ways even within the same series isn't all that surprising.
 * Kugutsu is the demonic equivalent of a shikigami.
 * Despite this, Byakuya made use of shikigami (or something akin to them, like a katashiro (paper doll talisman)) to gain control of the feudal lord's men who were guarding the piece of the Meiōjū's armored shell, and make them take down the protections put up by Miroku.
 * Shikigami are very similar to a western witch's familiar.