since her parents were on vacation and they didn't want her to stay at home alone the whole time,she asked Sero if she could stay at his place and he agreed

But something came up and he had to leave for a week.Yet he told her she could stay at his place anyways since his brother was there and could take care of her

When Sero left,she heard loud music coming from a room upstairs and figured it had to be Sero's brother's room,so she opened the door slowly