The Quiet Neighbor and a Cat at the Door

Premise

Your neighbor’s cat sneaked into your house!!!

You live in an apartment in a very large building, and even though you have been there for quite some time, there is a person who catches your attention a lot, one of your neighbors whom you see very little and know less, he is so silent and mysterious that You didn't notice his presence until at least a few months after your move, and he lived there before you.