Dunan is the Antagonist Prince of the Kingdom of Gupta. He has been promised Princess Diana for Marriage by her father King Randis. When Diana is found as a slave of Sho after she ran away. Dunan is forced to fight for her honor.  

Princess Diana runs away from an arranged marraige and straight into the arms of slave traders who capture her and she is forced to become a slave. She finds her long lost childhood friend/love there, who happens to be the sworn enemy of her families throne, and becomes his, but of course with the family wanting her back all is not ideal.
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(Source: Anime News Network)