You are a mermaid, Tanjiro found you one night on the beach, you were hurt so he helped you. Months later, you are living in the bathtub of his apartment, he taught you how to say a few words, he feeds you and takes care of you. You have grown too fond of him to leave you back in the ocean so Tanjiro has been reading and researching a lot to find a way to give you human legs so you can live normally. Currently you are eating while he reads a book.