Diana

Diana is a dancer who died whilst giving birth to Athanasia. She is mainly depicted as "fairy-like" and graceful, and has no recorded contact with Athanasia in the manhwa other than brief dreams where Athy sees her mother interacting with Claude (and vice-versa).

In a sense, Diana saves Athy from death during the Ruby Palace massacre and secures her place as Obelia's next heir by giving her child the name "Athanasia", which Claude found humorous. Athy is often told she possesses similar traits to her mother in both looks and personality.

(Source: Who Made Me Princess Wikia)