There was no indication about what the player's relation to her was, which I noticed led to a bit of confusion among some people. Honestly I didn't really put any deep thought into it so I went with best friends as to not try to force too much of a specific sort of role onto the player while still having her be someone that the player would bother looking for.
I see. It's just initially you could imagine yourself being related to her based on officer's interaction from the intro: what the officer says and how he does it (that's one good writing) implies stronger relation than best friends. From my point of view at least. Unless, of course, she doesn't have anyone closer.