There is no Sailor Sun. That role is taken by Metaria.Either she was a Sailor Soldier with the Star

There is no Sailor Sun. That role is taken by Metaria.

Either she was a Sailor Soldier with the Star Seed of the sun who got corrupted by Chaos to the point where she became an inhuman monster,
or she was always a monster who managed to steal and either permanently tarnish or destroy the sun’s Star Seed.

This is why Metaria is linked to the sun in both the anime and manga, and why its power wanes as Metaria’s grows.