Her grandmother. The crux of how she escaped went into that visit of hers.
It was supposed to be a short visit. To check on her granddaughter that her late son left behind.
That bastard stepfather of hers acted like a gentle and kind man upon seeing her. However, on the side, he threatened Serizawa-senpai that if she revealed something to her, he would start by killing her grandmother.
Naturally, she was frightened once more. But the chance came when the bastard had no choice but to give them some time alone.
Perhaps her gut feeling informed her that something was wrong with her previously cheerful granddaughter, so she talked to her about that stepfather of hers.
At first, she was worried for her grandmother so she flowered up her words to make him appear kind.
“Grandma caught my lie and noticed my trembling body. She checked my body and saw the bruises I still had at that time.” Serizawa-senpai continued with her story.