Otonari no Musume ni Itazura

The main character lives alone in an apartment.  One day, a single mother and her young daughter move in to the empty room next door.

By chance, he becomes friends with the cute, shy girl, and she spends time in the protagonist's room after school while her mother is still out.

Exhausted from P.E activities, the tired girl falls asleep while doing homework one day.

"With her dress in that position, I can see her underwear"

...so I reached for her while making sure she wouldn't wake up...