Manga and Crystal series
Keiko was killed in a fire that broke out when lightning struck her husband's laboratory during his research when Hotaru was 4 years old. The same fire also gravely injured her daughter, who was eight years old at the time. During her life Keiko possessed the Taioron Crystal, which would later be used by Hotaru to soothe her own pains, and eventually become a power source for the Death Busters. While Keiko herself only ever appeared in flashbacks, Hotaru still seemed to hold her in high regard, and possibly objected to Kaolinite trying to pursue Souichi Tomoe because of this.
First anime series
Keiko's backstory was not elaborated on in the story. However, she was shown with her face covered in shadows in a very brief scene when Hotaru recalled her happy memories.