Rei is the male protagonist or friend of the protagonist in Pokemon Legends: Arceus.
Game canon
If the player chooses Rei, he falls through a crack in space and time and ends up in the past region of Hisui (Sinnoh), where he becomes a member of the Galaxy Team. As part of the Security Corps, he is tasked with trying to complete the world's first Pokédex. If not chosen, he is an understudy of
Professor Laventon and is a member of the Galaxy Team Security Corps, helping Akari_Legends out with his Pikachu. Rei looks a lot like
Lucas, but it's unknown if Rei is an ancestor or descendant, or neither at all.
Pokémon the Series
In "Arceus, the One Called a God", Rei appears at the end of the special, showing a montage of him traveling through Hisui with his Pokémon, as well as completing the first Pokedex.