Question: who did peko pekoyama kill?
Answer: She has the title of Ultimate Swordswoman (超高校級の「剣道家」chō kōkō kyū no “kendō-ka”). She killed Mahiru Koizumi in Chapter 2 in order to protect her master, becoming the culprit of the second murder case. To see her acts of villainy, go to this link.