Boku no Elf Onee-san

A world as old as ours. Except in this world, elves and humans have always co-existed. At the Elf Academy where this unique anthropology is studied, protagonist Takuma Kirihara is a student, and his father Ryutaro is a professor. At home, one more person is counted as family, though she's not: her name is Sophia Arudisu Fairglyn. She is an elf who assists Ryutaro's research, and has been a lifelong friend to Takuma. Like brother and sister they grew up together, attending the Academy each day. Sophia and Takuma's friendship was platonic for years, until Ryutaro went overseas for several days on a research trip. It was then that they became lovers. It was the happiest Sophia and Takuma had ever been. But, a dark shadow crept upon them. This is the solemn tale of burgeoning lovers torn apart. [From DLsite English]