Claude initially blames Athanasia for Diana's death, since she died in childbirth. This is a source of great conflict for him at first as he can't bear to see Athanasia but can't bear to part with her either.

Who made me a princess Claude age?

Claude took the throne at 18 years old. He met Diana when he was 23 years old. When Diana died Claude was 26 years old.

Who is Athanasia's lover?

later on into the series, it was revealed that Ijekiel also had romantic feelings toward Athanasia as he felt love-at-first-sight on their meeting in the first timeline, which is seen when Jennette introduced him as her fiance to athanasia in that life.