Backlight

Also called a rim light, hair light or kicker, depending on how hard it is and where it sits. Its job is depth. Without one, a dark-haired person against a dark wall merges into it and the shot goes flat however good your key light is. Put it above and behind, angled down, out of frame and out of the lens. Any small LED will do. If it flares or halos too hard, pull the power down or flag it.