In the Alola region games, the duality of the sun and moon is central to the lore. While Solgaleo represents the "Sun" and acts as the emissary of the sun, Lunala represents the "Moon" and is the emissary of the moon.
Illuminating the Rikoto Pokedex: A Look at Solar Light & Lunar Dark Pokémon Solar Light and Lunar Dark solar light lunar dark pokedex work
Unlike a standard Pokédex, the Aevum Pokédex tracks: The Concept: Lunar Dark vs