^ tbh, Mihoyo's lores about Herrscher are pretty a mess now. Back in ggz, 3rd Herrscher is considered both Mei and not Mei. She is like the dark side of Mei that Mei had always kept inside until she becomes her ownself in the 3rd Honkai Impact.
About Kiana, the closest to a Herrscher personality is God Kiana of GGZ, while Sirin is cruel and arrogant, God Kiana is calm, emotionless to the point she only shed a tear after Mei's death.
GGZ dlc has an emotionless God Kiana x an overprotective yandere Herrscher Mei lol.