KoreSister

Korekiyo x Sister is the het ship between Korekiyo Shinguji and his Sister in the Danganronpa series.

Canon
''' WARNING: This section contains heavy spoilers for New Danganronpa V3. '''

Although Korekiyo mentions his Sister several times in his Free Time Events with both Kaede and Shuichi, mentioning that she was sickly and spent a lot of time in hospital, she made his uniform for him and that he learned anthropology thanks to her.

However, the truth behind their relationship is revealed once Korekiyo's double crime in Chapter 3 is found out. Korekiyo says that he and his Sister were lovers. But because he was her brother and lover and because of her constant visits in the hospital, she didn't have many friends. She eventually died. Remembering how lonely she used to be when she was alive, Kiyo decided to kill girls he thought were worthy of being his Sister's friends, believing that now that she's a ghost, her friends should be ghosts as well. He set a goal of making 100 friends for his dead Sister, thus becoming a serial killer.

Additionally, Korekiyo claims that during one of his seances, her spirit visited his body and has ever since been inside him. Several times during the trial, Kiyo takes off his mask, revealing he was wearing a lipstick underneath, behaves in a different manner and talks with himself. However, as Maki pointed out, this may be a case of delusions after the traumatic experience of his Sister's death rather than a case of a split personality or a ghost possession.

During his execution, despite being happy to see his brother again, Korekiyo's Sister takes part in the exorcism, along with Monokuma, to get rid of his soul. This might mean that she did not appreciate him killing girls for her.

However, with the reveal in the final chapter, it's unknown whether the relationship he had with his Sister was real or whether it was a part of his fabricated backstory by Team Danganronpa. It's even unknown whether he had a sister at all.