An Incubus is a demon in male form supposed to lie upon sleepers, especially women, in order to have sexual intercourse with them, according to a number of mythological and legendary traditions. Its female counterpart is the Succubus. An incubus may pursue sexual relations with a woman in order to father a child, as in the legend of Merlin.
Religious tradition holds that repeated intercourse with an incubus or succubus may result in the deterioration of health, or even death. In medieval times, the church would use incubi and succubi to explain sexual functions, which were taboo subjects.
Incubus appear in Ichigaya and Ikebukuro at night. They appear as support for the Succubus field boss in Shinjuku Babel's docks area and as field bosses within Suginami's Diaspora instance. Incubus appears as the final boss in the Deranged Scheme questline, as an apex in his attempts to seduce a Pixie.
Shin Megami Tensei: Strange Journey
"Originating in European folklore, the incubus is the male counterpart to the succubus. It ravishes women while they sleep, impregnating them and causing nightmares."
—Shin Megami Tensei: Strange Journey Compendium
Devil Summoner: Soul Hackers
"Originating in European folklore, the incubus is the male counterpart to the succubus. It ravishes women while they sleep, impregnating them and causing nightmares. Offspring born from such encounters are said to be evil spirits and witches. If one is possessed by an incubus, churches could perform purifications which could ward the incubus off."
—Devil Summoner: Soul Hackers Compendium
Devil Summoner: Raidou Kuzunoha vs the Soulless Army
"A demon originating in European folklore, the incubus is the male counterpart to the succubus. It ravishes women while they sleep, impregnating them and causing nightmares."
—Devil Summoner Devil Chart
Persona 4
"A male demon of European lore in medieval times. They visit sleeping women and have sexual intercourse with them; the resulting children become witches and wizards."
Alternate palette from Devil Summoner: Soul Hackers
Artwork in Devil Children Red/Black Book
Artwork from Persona 3
Sprite from Kyuuyaku Megami Tensei
Sprite from Last Bible III
Sprite from Card Summoner
Model in Shin Megami Tensei III: Nocturne
Model in Shin Megami Tensei IMAGINE
Trivia
In the original Shin Megami Tensei, Incubus was merely a palette swap of another demon, Leonard.
Incubus and Succubus are both depicted as wearing anklets with a bolt for Incubus and a nut for Succubus attached respectively in an innuendo reference.