Reiko Kokonoo is a 3rd year student of Onigashima High School and assistant to Shirou Hitsujikai. Her true personality is unclear as she changes it to suit her love interest's needs or desires. She is an antagonist in the series.
Though her personality depends on the guy she is dating, Reiko is shown to have a criminal side as stated in the anime. Her file reveals that she has been charged with shoplifting and has had a few run-ins with the police. Despite this, she has good grades and is a good cook.
Regardless, she does not seem to have remorse for her actions or the harm they may cause.
Unlike the heroine of the story, Reiko is quite busty. She has long blond hair that reaches mid-thigh and light brown eyes. Like most high school students her uniform is her main outfit, though Reiko's is modified to reveal her ample cleavage and long legs.