Riko Mukai is a character in the Ojamajo Doremi 16 light novel. She is friends with Hazuki and was the first to show her the Ura Site where someone uploaded rumors of her. She plays the violin and is the daughter of a company owner.
Riko's appearance so far is unknown. No detail has been said, nor has any image depicted her.
She is initially described as a frivilous girl who looked out for Hazuki since she began to attend the private school. It is possible that this is a fake personality though, as she was revealed to be the one who started all of the rumors surrounding Hazuki. She has a bitter-nature towards those of natural wealth after her fathers company went bankrupt, and hates feeling like she is lesser-talented than her classmates.
Riko was first described as a friend to Hazuki when the girls were trying to comfort her and asked if she had anyone at school supporting her. Hazuki explained who Riko was and the girls asked to meet her some time since she sounded like a "good kid", according to Aiko.
After things began to take a worse turn, Doremi, Aiko, and Hazuki went to the mall to look for bathing suits in hopes of luring out whoever was starting the rumors. As they observed sexier outfits, Aiko and Lala could wear someone's cellphone take a picture but they lost the figure to the crowd nearby.
Some days later, Riko had been feverish and Hazuki helped her to the nurses office after she fainted. There, in her sleep, Riko revealed the truth about what she had done. At first Hazuki didn't want to believe it, but the trio confronted her in the park a bit later. At first she denied it, but after the girls go proof Riko was forced to admit to what she had done.
She explains how she at first didn't mean any harm, having posted just to humor her frustration. But as she got to learn more about Hazuki she began to act out of spite until her goal had been to backstab Hazuki. By then she did realize she was wrong but she was unable to help herself. With that Hazuki asked why she did this.
Riko explained how her father owned a company had had gone bankrupt. Her mother took multiple part-time jobs while her dad tried to fix things and she was left to tend to the chores at their house every day. She had to attend public school during her three middle school years and missed out on many vital Violin lessons. Her father was able to get a new company started after some time of hard work and dedication, which was how she was able to attend Karen Girls' Academy starting with highschool.
After apologizing for what she did, Hazuki chose to forgive her and the girls remained on friendly terms.