“Getting killed is sooooooo violent, don'tcha know.”
Monokuma is a bear who acts as the headmaster inside Hope's Peak Academy. He is infamously known to be associated and controlled by Junko Enoshima, the girl who is responsible within the "Killing Game" itself. His most distinct feature is his Yin-Yang body, which represents both evil and good. However, many legends surround his real history and the real reason for his murderous behaviour, not that he is controlled.
Some legends say that Monokuma was once a teddy bear abandoned by his former owner. He was put next to an abandoned shrine that has now been populated by malevolent tsukimonos (spirits possessing inanimate objects). He was possessed by these spirits, causing him to change his colour from brown to some kind of Yin-Yang. This teddy bear, now named as Monokuma, prompting revenge to the owner who abandoned him day and night.
Others say that the spirit of a serial killer, who once murdered children in cold blood by using manipulation tactics such as offering them for a walk, had possessed into his own teddy bear inside his house. The teddy bear, who was originally white-coloured, had his body mysteriously transformed into the colours of Yin-Yang.
In the next few years, mysteries have faded away about Monokuma's true existence. Neither the students of Hope's Peak Academy nor even the Ultimate Despair, Junko Enoshima, know anything about his real history.
All they know and fear is that Monokuma is the embodiment of murder and manipulation inside his hollow body and uses them as tools for his own deeds.