Bertilde

Bertilde (ベルティルデ, Berutirude) the second daughter of Count Groschel, and Brunhilde's younger sister.

Spoiler from Part 4 Volume 8 Bertilde looks a lot like her older sister Brunhilde. Their similarities are so strong that Rozemyne thinks of Bertilde as a "mini-Brunhilde" upon seeing her for the first time after her long sleep.

Bertilde and her sister Brunhilde are the daughters of the third Giebe Groschel and his first wife.

Spoiler from Part 4 Volume 8 Like her older sister, Bertilde is planned to become Lady Rozemyne's attendant once she starts attending the Royal Academy next year. For this purpose Lady Elvira already started to train her.

Upon their return from the royal academy after Rozemyne's second term, Brunhilde introduces her little sister to her future lady in the winter playroom.