Yoon Bum's Grandma is a minor character in Killing Stalking, only featured in Bum's flashbacks as for now.
She is an elderly woman with grey hair.
She's kinder than her son, but passive. Due to her age, she can only watch her Son torment Bum, but she is shown to try to defend him when things go to far.
Unknown whether intentionally or not, she told Bum to sleep in his uncle's room one day because the uncle wouldn't be coming home at that night which turned out to be a lie since he came home, went to his room and abused Bum with a belt, then raped him.
This shows that at the point when the abuse became worse and the rape began, that she rather have her son beat up Bum than herself, whether she knew about the rape or not is still not known.