You may not notice but her name is actually mentioned once in the whole series of the Anime (and maybe in the manga [please confirm]) by Yoshitake's older sister as "Mei" at episode 7,at an Arc Titled "Highschool Boys and Siblings" (Rough Translation please confirm).
Her personality is very sadistic, she has the strength of approximately a teen gorilla and is most likely targeted for pranks by the protagonist trio.
She is short tempered and wears her hair in twin tails. Her approximate age is 16 since she is at 10th grade (Please Confirm) other measurements such as Bust, Waist and Hip remains a mystery.
She sometimes shows soft spots to her older brother when the time is right (Although this is based on episode 12 [final] which has the probability that it was solely Tadakuni's dream and so this fact remains a blur, meaning she may never even show a soft spot to Tadakuni).
She only talks to her brother about once a month (as mentioned in episode 2 arc: highschool boys and childhood) she and her brother have a distant relationship meaning she practically "hates" him.
Her panties appearently always gets stolen by boys mostly from the main trio (Hidenori, Yoshitake and Tadakuni) but it also got stolen from Hidenori's older brother Yusuke in episode 7 Arc: brother.
She tends to get overexcited when anything good befalls her (example: Episode 8, Arc: Highschool Boys and Tower where Mei throws her bag far up a tree. Note: this Arc also features an example of her monstrous strength)Information may not be complete please add more if so. -Rangeki Akibayashi