It's not just coincidence that Mayumi sounds like Biri-biri and looks (and drinks) like Mikoto's mother Misuzu. As it turns out, Mikoto is not really an electromaster, espers AREN'T real, everything was down to her middle-school 2nd year delusions. It was all her wild and inventive imagination. Eventually, her friend Touma was able to break her illusions quite literally, bringing her back down into the real world. They dated for a while, but this only served to drive Kuroko away in a fit of jealousy, and Touma's continual (and apparently hereditary) wandering eyes eventually drove them apart too, driving her to drink. Now she flounders aimlessly, continually dating the same kind of two-timing men like Touma and keeping friends with an auburn-haired twin-tailed Psycho Lesbian as a feeble attempt to relive what she feels were her glory days.
WMG / The Kawai Complex Guide to Manors and Hostel Behavior