Gaki are ghosts tormented by a ravenous hunger for the lifeforce of humans. They only appear at night, usually as small clouds of black smoke or balls of cool fire. They can use shapeshifting to assume a human form, though it will rarely be the form the gaki had in life. They may also show a monstrous visage to frighten their victims.
In Zen monasteries it is customary to make a small food offering to the gaki before beginning to eat a meal.
Relevance to Urusei Yatsura
Takahashi has claimed that the character of Ran was loosely based upon gaki. Like a gaki, Ran seems to change from a pleasing visage to a scary one continually. When her frightening side shows she seems almost demonic with fangs and pointy ears. At moments like these, the way her hair even moves like a nebulous cloud is related to the idea of a gaki's insubstantial form.
Also her rather vampiric power to suck the youth and vitality out of people is similar to the way gaki are said to feed on humans.