I read the novel long ago so some details might be wrong,
Renei's mother was from a certain ancient tribe/clan (idr) and have the same power as renei, the thing is they cannot live past a certain age, the mother was already pass that age when they met mc and was in constant pain and suffering, she was only alive for renei sakes, as they live somewhere remote. Mc was passing through their house when they met. Here they also met the senior guy. He was passing through renei's house or something.
When renei's mother couldn't bear the pain she begged mc to kill her unfortunately when mc was killing her both young renei and that guy witnessed it so mc eventually completely wiped off renei's memories. Idk why he didn't wiped that guy's memories.
SOMEONE SPOIL AS TO WHAT REALLY HAPPENED BETWEEN THE PROFESSOR AND RENEI'S MOTHER. TS JUST GOT AXED, I CANT BELIEVE IT ╥﹏╥