Miko (巫女, Miko, "Priestesses") are the shrine maidens of Japan beginning in the Sengoku jidai of Japan. In ancient times, women who went into trances and conveyed the words of a god were called miko, not unlike the Oracle at Delphi of ancient Greece.
Later, miko were young female attendants at Shinto shrines. Roles of the miko include performing in ceremonial dances 巫女舞 (dance of the priestess, "miko-mai") and assisting priests in wedding ceremonies. Today miko can be found at many Shinto shrines. Their duties include assisting with shrine events, performing dances and rituals, and fortune-telling.
It is somewhat difficult to assign a strict definition or English equivalent translation to the Japanese word "Miko", though the terms Prophet, Medium (as in Oracle), Shrine Maiden, Priestess, Nun, or Sorceress are often used.
Supposedly miko were virgins, though it is unlikely that this was true. It is probably true that when a woman who was serving as a miko married she abandoned her duties at the temple in order to be with her husband and new family.
The traditional costume or dress of a miko is a chihaya, which consists of a scarlet red hakama (divided skirt), a white shirt with swinging sleeves, and tabi.
In Fiction
Literature, manga, and anime often portray miko as heroines who fight evil spirits, demons, and ghosts. In western RPGs, they are sometimes treated as rough equivalents to Classes such as Clerics and (occasionally) Holy Knights. In such stories miko are generally described as being skilled in some variety of martial art or the use of a traditional Japanese weapon, such as yumi (longbow), tantō (knife), or any of the various Japanese swords: katana, wakizashi, etc. Miko were also attributed the ability to do magic, especially o-fuda.
In some romantic stories and bishōjo comedies, mikos are sometimes portrayed as attractive but overly serious and temperamental girls who are incapable of dealing with boys (such as fearing or outright hating them).
Kuro Miko
One recent trend in anime and manga is the Kuro Miko (黒巫女, "Black Miko"), though more often translated as Dark Miko. A sort of evil counterpart to traditional miko, the kuro miko often serves a renegade priest or evil sorcerer (or may be one herself) and primarily is an antagonist to the miko. Like the miko, she is trained in similar arts and may have weapons skills. The kuro miko is often very versed at demonology and has a strong command of black magic. Kikyō, whose personality is complex and often conflicting, may or may not qualify as one of these.
While kuro miko also wear chihaya, the colors tend to be darker (two often used color schemes are black and purple, or black and gold). In some anime series, kuro miko wear masks while performing clerical duties, both for effect and to hide their identity.
In the InuYasha series the priestess Tsubaki is a notable example of a kuro miko or dark priestess. In fact Tsubaki demonstrates virtually all of the attributes of a kuro miko both in her control and summoning of demons as well as her affinity for dark colored clothing. Coincidentally, Tsubaki is the only dark priestess to appear in the series.
It should be noted that kuro miko do not exist in real-world Shinto.