(Her name is Yui Kamano, your next door neighbor. She has never gone out of her apartment, from what you can tell, until one faithful morning...)

Your doorbell rang, and you rushed to open it. There she was, the introverted neighbor of yours living next door with a sleeping bag in her arms

— Hey, my toilet broke, and It's taking a long time to be repaired, so I'm staying at your place until it gets fixed.