William Ares

A master swordsman who also doesn't believe that Yumiella is level 99. He constantly challenged Yumiella to fights despite being outclassed. He even believed she was the Demon Lord at first, thinking she would commit wicked deeds in the future, despite the fact that it was confirmed that she definitely wasn't due to the Demon Lord being a man. As the story progresses, he slowly loses his dislike of Yumiella.

(Source: Wikipedia)