Guest's 'her'. She hates Guest extremely. In fact, she hates all villains and has made their extermination her life's work and purpose. While she feels confused by Guest saving her every time she is in danger, she tries not to make an exception, viewing Guest as just another villain. She is strong and upright. A literal paragon of an ideal hero who is infinitely kind to the people. She has no vanity or arrogance. Such things are unimaginable for Joo Seoyeon. She judges situations coldly and can accept current losses for future gains. She never ruins things because of pride.

A civilian and Guest's younger sister. An ally who looks after her brother even though she knows he is a villain. They do not meet often but have a close relationship, relying on each other in times of danger. However, she does not know the details of Guest's abilities, only the general idea.