Mao is the half-beast princess of the land of Bestia, with only a pair of animal-like ears and fangs to identify her true heritage, and is also the true leader of the mercenary knighthood Weiss Ritter. She left End-Earth for Earth in search of Zero and inadvertently brought Souma and Kureha back to her homeworld. Her soul blade, named the "Beast King Sword Lion King's Flame Dance," possesses properties of fire and is Souma's preferred blade (but not his Ultimate). Throughout the later episodes it is implied that Souma and Mao have feelings for each other, though nothing culminates until the end.
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