Han Sung Yu

Yu Han-sung is an Advance Ranker and the Test Director of the Second Floor in Season 1. Despite his feminine looks, he is actually a male. He has a pretty big reputation as a ranker because he rejected to be a High Ranker, and is well-known as a genius Wave Controller.

[Spoiler] He is a key member of FUG in taking Viole to the top and the main antagonist of Part I. He plays an important role as character because he is close with the main story of Tower of God. He had since left his position as the Test Director. [/Spoiler]

(Source: Tower of God Wiki)