Princess Sissy is the final boss of Power Instinct Matrimelee. In the Neo Geo console version of the game, she could be accessed by holding all four buttons down when selecting the character and before the Vs. screen. She reappeared in Shin Goketsuji Ichizoku: Bonnou no Kaihou, but this time was no longer the final boss and the only way she could be fought as an opponent was through the game's Mission Mode, or the Training Mode.

Due to her older brother not being the ideal image of a King, Sissy's father, the King of Certain Country, decided to train her to be the successor of the throne. But as a Queen needs a King, Sissy's father decided to organize a martial arts tournament to find a strong warrior to continue the lineage of the royal family. Princess Sissy later became sad as the tournament was a failure. She just couldn't forget the excitement of the Matrimelee tournament. The King, worried about the emotional state of his daughter (who is usually joyful and vigorous) invited the world's strongest fighters to his country to convince them to enter a new tournament, the "Bonnou Kaihou" tournament.

This time the prize is anything that the winner could wish for (except the throne succession). When the King told Sissy about the Bonnou Kaihou tournament, her characteristic smile returned to her face, finally she will once again have that feeling that is present only in battle.