WMG / World's End Harem

Kamiya Karen is Doi Mayu.

Nothing is known about Mayu other than her delicate constitution and her love of Shota, and Karen is still a mystery besides her efficiency at handling Shota's high school life. Their hair looks similar, and Mayu would have a good reason to act like Karen while saying nothing to Shota: to give her brother the fun high school life she feels he deserves after all the bullying he suffered before going into cryostasis.