Keiko is the best friend of Rinko Nikaido and Tamaki Maekawa. In the anime, she is shown to be a close friend of theirs from the start, but in the manga she is bullied initially by Tamaki and has money extorted from her on a regular basis.  Keiko often appears on the sidelines of matches offering support.
Keiko is one of the best friends of Rinko Nikaido. In the manga, she is bullied by Tamaki in school, and almost ended up dating The Rigani for money so she could pay off the bullies. Rinko steps in to stop her, which leads to The Rigani fighting Kinniku Mantaro.
In the Ultimate Chojin Tag Arc, she stayed behind with Tamaki. It's unknown if these two know Rinko sneaked on board the time ship. We mostly see Keiko alongside Tamaki and Rinko in the series, where she provides support as a wrestling fan to various teams.
Keiko is introduced as a student of Shirobara Private High School, an all-girl private school, and she is a member of Class A and a classmate of Rinko.  We see she is being bullied by Tamaki, who expects 30,000 yen from her, and she has been forced to steal money from her mother to make as much as possible. Tamaki attacks her, and she is saved by Rinko, whom she rejects help in the following scene.  Keiko then leaves - followed by Rinko and Mantaro Kinniku - as she considers Tamaki's suggestion of 'compensated dating' to make money. 
Keiko agrees to a date with a man that offers her 40,000 yen should she indulge in his fetishes, at which point she panics and Rinko comes to her defence, and this allows her a chance to escape.  We do not see Keiko again for this filler arc, but we can assume she made it to safety. It is revealed the man she agreed to date was The Rigani in disguise. 
In the anime, she appears with Rinko and Tamaki during the Tel-Tel Boy fight, and in the same episode, Mantaro mistakes Rinko calling out to Keiko as cheering. During Mantaro's fight with Check Mate, Keiko realises the one vote called in for Mantaro was by Rinko, which Rinko denied. Along with Rinko and Tamaki, Keiko cheers on Team AHO for the majority of the anime and manga. She is the second hostage up for grabs in the Poison Six Pack Arc, and she is saved by Gazelleman and Seiuchin, with Seiuchin catching her.