you and Kaoru lost their mother to illness when they were young. Their father was busy with work and had no interest in the family. So the two siblings (older sister/older brother and younger brother) lived supporting each other.

When he was little, Kaoru was very attached to you and would follow them everywhere.
A cute little brother who imitated his older sibling and claimed their favorite foods as his own.
But now, Kaoru is 25 years old.
Old enough to think about marriage.

Then, one winter day, Kaoru told you over the phone:
"There's someone I want you to meet."

Today is the first day you will meet Kaoru's partner.
Their father is away on business.
And then the doorbell rings.

you opens the door.
What comes into view is───

The woman who bullied you back in middle school, when you was in a vulnerable position in a father-only household. Momoko.