There's an apartment building we can enter at the right of the cinema (near the edge of a collapsed street). When we climb the stairs up, there is a door which is locked. Where do I find the key?

Thanks

Moved your topic here since its asking for game help.

Sadly I haven't checked there yet, so I can't give an answer :c

It's a part of the Ribbons sidequest.

Solution:

do the ribbons sidequest

Thanks, this is the only sidequest I haven't done. I should have thought about it earlier.