電車の中とかで音楽聴いてたら思わず口づさんじゃう系女子、ミズキちゃん！

この後ふと我に返って赤面するまでがデフォ

Mizuki was listening to music on the train and unintentionally started singing along.

After she realized what she was doing, she blushed and went back to normal.