Preta (प्रेत in Sanskrit, Peta in Pali, Yidak (ཡི་དྭགས་) in Tibetan, E-gui (餓鬼) in Chinese, Gaki (餓鬼) in Japanese, usually "Hungry ghost" in English) is a spirit that originates from Buddhist and Hindu texts. Generally, they are the spirits of greedy, jealous or corrupted people who have become, for their negative karma, demons with an insatiable appetite for a particular substance, that can vary from corpses to anything bizarre. In some Buddhism religions, Preta is the name of one of the Six Realms that represent the Six States of Rebirth. In the Chinese traditional religion, they are beings driven by intense emotional needs in an animalistic way. In the modern Japanese language, spoiled children are called "gaki", as a reference to this demon.
Preta are described as human-like spirits that are usually invisible to the human eyes depending on their mental state.
They have sunken, mummified skin, narrow limbs, an enormously distended belly and a long, thin neck; their appearance, as so, reflect their state of insatiable hunger.
In Japanese art they are represented with bulging stomachs and small mouths and throats or as balls of smoke or fire, and are shown, usually, licking spilled water in temples, accompanied by demons that represent their agony.
They dwell in the wastes and in the deserts and usually have problems while dealing with their hunger and end up becoming even more hungry: someone can eat a little, but have difficulties in finding food or drinks or have problems swallowing food; some even find that what they eat seems to burst in flames or see the food they desire vanishing before their eyes.
In Hinduism, their body is composed of Air and Akaash (comparable to aether, black matter or space), and since they are missing three elements (Fire, Water and Earth), physical intake and digestion is not possible for them.
They usually suffer similar conditions to those in hell, but they can freely move instead of being confined in a subterranean world.
Preta are usually not seen by humans as a nuisance unless their hunger is for blood, though in some other traditions, they prevent others from satisfying their own desire. Many times, though, they are seen as beings to be pitied and, for example, in some monasteries monks leave them some offerings before meals; some Japanese Buddhists also dedicate a day to remember them, as they believe that such offerings and remembrances will, someday, free them from their endless pain.
"Origin: Japan (sic). The forms of greedy souls. They have large heads and a thin neck, fat bellies, and shriveled limbs. They are plagued with a hunger that can never be satisfied."
—Shin Megami Tensei GBA/iOS Dictionary
A Preta is the very first demon that the protagonist actually encounters in Shin Megami Tensei, where he attacks a civilian in the Kichijoji Mall and then proceeds to attack the protagonist, leaving the scene after inflicting substantial damage.
This event also leads the hero to obtain his first weapon in the game from the dead man's body—an "Attack Knife"; and his getting wrongly arrested (the fan translation specifies that he was framed by the police for the murder) that leads to his temporary incarceration and meeting the other main characters.
"The form given to those who are cast down into the Preta Realm, one of the Six Realms of Buddhist scriptures. Those who lived extravagant lives are said to be cast into the Preta Realm after death. Preta are constantly tormented by hunger and thirst, but whenever they put something to their mouths, it bursts into flame, and so they are eternally deprived of food and drink. Their figures are gaunt, except for their bellies which are strangely swollen."
—Shin Megami Tensei IMAGINE Compendium
Gaki can be contracted in Ueno, Suginami, Shinjuku Babel's docks area, Old Tokyo Metro, and the bronze level of Ichigaya Camp. It appears regularly as a tutorial boss and an enemy mob in Virtual Battle, Kuchinawa Cave, the Ichigaya Gold boss room, and some rooms in the Second Home instance. It also plays a central role in the Superficial Contract quest.