Futakaku Kankei双角カンケイ。 (Japanese); 双角关系 (Chinese); Bicornuate Relationship (English); Soukaku Kankei.

From Kouyuri: Twin sisters Airi and Himari have always been very close, and worn identical hairstyles, clothing and accessories. But after they enter high school, Himari starts to drift away from her sister. When cornered by Airi, she explains that she has "a promise with senpai to fulfill," no matter what.