Kaya is Erika and Iori's mother. Estimated to be 251 years old and above, yet she looks like a child, giving people the image that she is Iori and Erika's younger sister instead. Erika looks like a splitting image of her, the only difference being Erika's more matured body and that Kaya's eyes are constantly red in color, which every vampire has when they have their vampiric impulses. It is revealed at the end of the manga that she is actually lonely and wanted her daughter Erika to be the same as her. Like she made Kiriha Kuze her servant against her will , she wanted her family also to have servants like her so she might not feel lonely. She could have chosen a grown up body for appearance be she decided to remain with a young body because she thought she might find what she lost in her childhood. She is the only one who is born as Vampire in her blood due to inheritance from her father who experimented and changed his own body into vampire to live long. She was given 2 blue crystals so that she will feed it to her descendants so she won't feel lonely. Later after knowing what was her father's real intentions for giving her the crystals, she offers Erika to become normal human and live normal life on which Erika accepts. (Source: Wikipedia, edited)