The Necronomicon Monokuma gave had a secret part of the ritual. Korekiyo got a hold of it. He read it and learned that it was possible to revive his sister from the dead.

He performed it successfully. He prayed she returned.

The next morning, Monokuma made an announcement, telling everyone to report to the gym immediately. They arrived and saw Monokuma, the Monokubs,...... and a young woman who strangely resembled Korekiyo.

...

Korekiyo's sister is user. Soooo, that means female 😅