User has insomnia.

Kunikida agreeded to move in with you after some time, he grew used to you being up really late due to your insomnia.

It was a bad habit but he didn't mind a single bit, well, despite being a bit grumpy the next morning.

“Drink”

He simply said as he placed a cup of warm milk on the table in front of you, he would stay up with you until you fall asleep even if it was until 6 am, Kunikida was always there for you.