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.
"Originating in European folklore, the incubus is the male counterpart to the succubus. It ravishes women while they sleep, impregnating them and causing nightmares."
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.