Hemera (day) and Aether (light) were two of the children of Nyx and Erebus, though some versions of the myth claim they are the children of Chaos instead, or the children of Nyx and Chronos or jut Chronos. They don’t appear in a lot of myths because while you might hear these gods referred to as personified concepts, they’re not like other gods down the line. Hemera WAS day, not the goddess of daytime. She didn’t just represent her element, she was that element, and concepts like day and light are kind of hard to come up with active stories about.
One thing that is known is that Hemera didn’t spend a lot of time with her mom, the two would greet each other at the end of every night when Nyx returned home and Hemera was on her way out the door.
Aether is called the god of light or brightness, but that’s kind of a misnomer in the sense that a modern person would think of light. Technically he’s the god of the bright, glowing upper air that only gods breathe as opposed to the gross lower air reserved for us mortals. During the night, Nyx separated the flowing divine air from the mortal air with a veil of mist (which makes me think of Mistborn). During the day, Hemera dispels the mist so man can see the divine light of the gods.
Being a very powerful divine couple that can’t perform an active role in most myths reduces gods to impressive names to throw around in divine lineages. Hemera and Aether might have been parents to Thalassa (a primordial sea goddess), or they might have been parents to Thalassa, Uranus, and Gaia. Or Aether might have hooked up with Gaia too and produced “Grief, Deceit, Wrath, Lamentation, Falsehood, Oath, Vengeance, Intemperance, Altercation, Forgetfulness, Sloth, Fear, Pride, Incest, Combat, Ocean, Themis, Tartarus, Pontus; and the Titans, Briareus, Gyges, Steropes, Atlas, Hyperion, and Polus, Saturn, Ops, Moneta, Dione; and three Furies – namely, Alecto, Megaera, Tisiphone.” (thanks Theoi.)