Monokuma

Monokuma is the main antagonist and mascot in the official Danganronpa series. He isn't a character ever interacted with in Danganronpa: Leftovers and is said to be "doing his own thing." He is an important part of multiple executions, showing he is aware of whats going on in Monokin's Home for Refugees and Despairs.

Further explanation of Monokuma as a character will be here