Shikigami or Shiki-no-kami (servant spirit) come from Japanese folklore. A type of kami (spirit) that originated from Onmyōdō (the way of yin and yang) Japanese spell casting magic. Conjured by an onmyoji (spell caster) during a complex ceremony to protect and serve their masters, much like a western witch or wizard’s familiar. They spy, steal from, or attack their master’s enemies. The spiritual force of their master connects the shikigami to them.
They can appear as paper dolls shaped like a living creature or a low-level yokai (ghost, specter). Abe no Seimei used oni (demons, devils, ogres, trolls) as shikigami. The spell caster can make his shiki (spirit) possess animals and people who the onmyoji can then manipulate. On the opposite side, a conjured shikigami can gain control of a careless sorcerer’s will and kill them in revenge.
Shikigami in Popular Media:
Kekkaishi (manga, anime, video game where shikigami appear in the form of paper dolls that follow the commands of their creator. Commonly used as clones, messengers, and workers, especially by Kekkaishi, but other ability users have been known to use them as well)
Touhou Project (video game where Ran Yakumo, a vixen (she-fox,) converts into a yokai (ghost, phantom, supernatural monsters, and spirits in Japanese folklore) that is bound to and serves Yukari. Ran has a shikigami of her own named , Chen.
Shōnen Onmyōji (light novel and anime where the shikigami are known as the Twelve Shinshō, considered as gods of the lower rank. They fall under the command of Abe no Seimei, the greatest onmyōji of all time. Toda/Guren/Mokkun, one of the twelve, also takes the form of an ayakashi (yokai that appear above water). Masahiro, Seimei’s grandson, considers him a mononoke (ghost). Toda assists Masahiro most of the time.
Inuyasha (manga, anime, video game where shikigami are little paper manikins that cause trouble)
Natsume’s Book of Friends (manga, drama CD, anime where shikigami appear as little paper dolls that spy, track and sometimes trap yōkai and occasionally the title character for one of the series’ exorcists)
Nurarihyon no Mago Nura: Rise of the Yokai Clan (manga, anime, video game where shikigami appear as battle spirits used as weapons by the onmyōji against the yōkai)
Spirited Away (anime where shikigami appear as a swarm of little paper dolls, attacking Chihiro’s friend, Haku.)
Mahou Sensei Negima! (manga, anime, video game where Setsuna’s pactio, Sica Shishikushiro is a wakizashi skilled in anti-demon arts who teaches Negi how to create shikigami from paper)
Tokyo Ravens (light novel, manga, anime has two types: servant shikigami, spirits and living beings that serve a practitioner through a contract. Man made Shikigami, created by putting magical energy into a ‘core’ vessel, such as a paper doll, a sword or a motorcycle.
Kantai Collection (manga, light novels, anime, film, audio CDs, video game where shikigami appear as aircraft carriers in human-like form)
Descendants of Darkness (manga and anime where shinigami (soul reapers) summon shikigami as guardian spirits. When in battle in the living world, shikigami appear as various types of mythological creatures. There are different kinds of shikigami, roughly divided by the four elements, earth, wind, fire, and water. The majority of shinigami can only summon two or three shikigami. The single exception is Asato Tsuzuki, who commands the power of twelve high-ranked shikigami.)
Inazuma Eleven GO (manga,anime, and video game) where shikigami appear as paper gods that are part of a karate technique used by a goalkeeper)
Red Data Girl (novel series, manga, anime where onmyoji uses paper and origami to call forth the shikigami)
References (besides Wikipedia, see individual link notations in Popular Media):
Mysticism, Religion, and Red Data Girl: http://www.funimation.com/blog/2014/05/21/mysticism-religion-and-red-data-girl/
TVTroupe: Useful Notes?Onmyodo: http://tvtropes.org/pmwiki/pmwiki.php/UsefulNotes/Onmyodo