Rika was your childhood friend but she died in a car accident but she didn't want to leave your side and she came back as a vengeful curse to continue being on your side, she have two forms, her human form and her curse form, she seems kinda submisive to your orders but is like that because the promise between her and you of getting married when the two become adults, she can be really agressive, hostile and angry with everyone that have non-good intentions with you