The Greek god who is the personification of the upper air and light is Aether.
Aether represents the bright, pure air of the heavens—the substance ancient Greeks imagined above the ordinary air breathed by mortals. He belongs to the earliest generation of divine personifications, or primordial deities. In the account associated with Hesiod, Aether is the son of Erebus, darkness, and Nyx, night; his sister is Hemera, the personification of day.
The distinction between Aether and other primordial figures matters. Chaos is the primal gap or disorder, Tartarus is the deep abyss beneath the Underworld, and Erebus personifies darkness. Aether instead signifies the radiant upper atmosphere surrounding the gods. The related Greek word later became important in philosophy and science, where “aether” was imagined as a substance filling the cosmos.
Aether is not the same as Zeus, Helios, or Apollo. Those gods can be associated with sky, sunlight, or illumination, but Aether is the divine embodiment of the luminous upper air itself. His role is therefore abstract and cosmic rather than that of a ruling Olympian deity.