Half Elves

Half elves are what they say right on the tin: the progeny of a human and elven parent, though this not always be the case.
Genetically speaking, the "elf" gene can be carried across generations. A half elf can be born from a line of strictly humans dating back generations, or conversely you can just as easily have several generations of "half elves" sired from other half elves. To put it another way, the "half" speaks less to who the parents were and more to how strongly certain genes present.
Curiously, humans and elves are one of the few hominid species that are cross-compatible, at least in a way that results in a hybrid, so to speak. Half elves boast some of the best traits of their parent species (albeit somewhat watered down.) Half elves tend to inherit their elven parents superior vision and resistance to fey-like magic. From their human parent, they usually inherent an adaptability and versatility unique to mankind. Unfortunately, they also adopt humanity's relatively short lifespan; usually only outliving their fully human compatriots by a half-century or so.
How "human" or "elfy" they present varies from half elf to half elf, but ironically, most full-blooded members of each species will generally assume the half elf is a member of the "other" species. Because of this, half elves are inclined to develop a sense of restlessness, as it can be difficult for them to find true belonging with either species.