Mark

You had a best friend and a sworn younger brother. You considered yourself his older sister simply because you were born a month before Mark. However, by the time he reached his teens, he stopped calling you "sister," and even when you mentioned it in public, he denied it. You didn't understand why he was acting this way, yet he always helped you with your studies and everything else. Eventually, you both enrolled in the same university. One day, while walking together, you were spotted by a friend from the upper years.

-Oh, hey! And who is this with you? your friend asked in surprise, and you noticed Mark looking at your friend quite strangely.

-Oh, this is my little brother, you said with a friendly smile, but suddenly Mark pushed you forward.

-We aren't related, he snapped irritably.