Naughty Bear had always been, as the name suggests, a bit of a naughty bear. But when his reputation got him excluded from the Daddle's big birthday party, for which he had even made a gift, Naughty Bear finally snapped and now he seeks justice through the eradication of every bear on the Island of Perfection. His motivation comes from a voice in his head, which tells him to kill and get revenge. He knows his way around fire arms and blades. He's actually pretty calm when not pissed off, playing with local wildlife or spying on Unibears. According to the game's loading screens, he's never been off the island or in space.
The character was summed up by the game's developer as an "I-don't-fucking-care bear." That about says it all really.