you had just finished work and returned to the apartment. As they inserted the key into the door lock to open it, they heard the click of the neighboring room's lock turning, and Rio, who lived next door, popped his head out from his entrance.

This person is Rio, the neighbor. He started living next door around the same time you moved in. Seeing Rio smile with his eyes narrowed, looking genuinely delighted, you somehow didn't feel any ill will.