The dialogue box and menu are (a) FRIEND(ling) Theory
So for a while now I have been thinking about how similar the dialogue box of Undertale and Deltarune (Light World) matches FRIEND's whole shtick.
As you can see here the Undertale dialogue box matches FRIEND basically 1:1 (besides the Spare being alternating for obvious reasons).
Another reason for why this theory could be true is the Sword Route.
As we know, the Sword Route is a metaphor for both Undertale and Deltarune, showing both the Genocide and Weird Routes. The device used to play it and thus the interface isthe OddController.
It literally says that the buttons and pink and yellow just like the dialogue boxes and it being tied to the metaphor to Undertale and Deltarune, and FRIEND is pretty telling that the actual box is also FRIEND-related or straight up them.
Now speaking about Undertale. There is a very very important non-dogchecked room:
A FRIEND with a creepy smile? Wonder who it is… Of course it could be flowey but I doubt that he can access secret rooms outside of the actual game. Furthermore, there is no one talking to us (at least anyone who's visible). It could very well be that the dialogue box, being FRIEND, is talking to us through itself about itself.
This is really grasping at straws but guess where the stats/menu screen is. To the left of the player character. The player character mostly faces to the right because that is the direction where they need to go. This means that the stats/menu screen (FRIEND) is behind you.
That's about it.