Ballos is the hidden ultimate antagonist of the game Cave Story, met during one of the difficult endings.
Ballos is the creator of the Demon Crown. His niece (his sister Jenka's daughter) Misery forced him to create it. Unfortunately, she was quite rude and harshly demanding. Due to this, Ballos cursed and doomed her (and also Balrog, but we don't know how he got into this mess) to obey the one who wears it.
Ballos originally had a very peaceful life on the surface many years ago and possessed a great talent for magic. He helped many people, who loved him in return. However, the jealous king captured him and tortured him non-stop, angry that all of his subjects viewed Ballos as the true figure of leadership in the kingdom. This cruelty continued until he went insane with rage at his weakest point. Ballos broke free of his prison and swallowed his once lovely kingdom in flames. Besides the king, he sadly killed his son and wife. Unable to look away from the sight and broken by what he had done, Ballos could only let out a crazed laugh.
Jenka, Ballos' sister, imprisoned him in the Seal Chamber in the Mimiga island. For Jenka, killing her younger brother would be impossible. Unfortunately, his magic continued to grow even stronger with each passing minute and supply the Demon Crown with the ability to endlessly resurrect itself.
In the Nicalis translation, Balrog refers to Jenka as "Auntie," thus implying that Balrog may be Ballos' son. This may be why Balrog is a servant of the Demon Crown.