Ada Vessalius is Oz's little sister who adores him very much. After 10 years she attends Lutwidge (ラトウィッジ), a prestigious school for nobles, of which she is a prefect. She keeps in contact with Oscar via letter and causes Oz and his friends to sneak into Lutwidge as she wrote that she has found a person she likes. It is revealed that she was the one who gave Gilbert his precious hat, that made Oz and Oscar think that Gilbert is the one that Ada likes. Then she asked Oz to let her be the one to return Elliot's bag, but when Oz said that he will return Elliot's bag by himself, Ada was slightly disappointed. However, in the later chapters she is seen on a date with Vincent. She has a cat named Dinah and later another one named Snowdrop. It is revealed later, much to Vincent's astonishment, that she has been studying about the Abyss and the occult in order to find a way to bring her brother back from the Abyss, but, during her studies, she has grown an interest in the occult.