Aether

Aether is a mystical gender-neutral name of Greek origin. In Greek mythology, Aether was the son of Erebus and Nyx, but shines bright despite parental associations. Erebus as the personnification of darkness and Nyx as the feminine personification of night, their offspring somehow became the personnification of bright upper sky. If you're looking for adorable, mythically-based, and unique twin names, Aether and Hemera would be an excellent choice. Hemera, as Aether's sister, became known as the personnification of day. With all of these associations in place, you can put a name baby's sunny disposition.