When I first saw this scene, it's really strange to see why Nero is always staring at things in front of her. If she was talking to Hakuno, she should see her left side because that's where Hakuno stands.

I get more confused when I saw the animation during this stage, Nero talks to someone.
The way SHAFT showing the storyboard is really strange

When I saw this scene I thought Nero is looking at Hakuno (I really did)
However, the next scene of Nero starts talking is even much more confusing. Because the camera doesn't tell us whom she is talking to. (If you want to show two people talk to each other and who is talking to whom directly, the ordinary photographer will picture two people into one scene like this.

This is just an example, you can clearly see two people are talking to each other.
However, the next scene is showing that Nero is still staring at things in front of her and it seems like she is talking to the air. (Look at the background, it's almost the same angle and its symmetry which suggests Nero doesn't turn to Hakuno's side nor look at him. If she does, she should at least roll her eyes to the left so we know she is looking at Hakuno)

If I picture this scene in this way you will know what I mean to.

So it really appears she faces nobody right?
The next scene shows Mumei and suddenly, all strange storyboards seem to make sense.

Then the next scene, if you watch the anime carefully you will see Nero apparently doesn't pay attention to the Hakuno stands beside her. She is still staring and walking to the things in front of her.
Then the last scene, watch the anime again, did you see it?
Why the background has to be symmetry? SHAFT played a small trick here
Normally, if you want to picture two people standing in front of each other, you place the camera on the side of them and take the picture, because it directly shows the relative position of two people.
However, in this case, the relative position of two people (Nero and Mumei) is hidden cleverly. Because the camera is placed
IN THE MIDDLE OF them.
So, it looks like this
How do we know the relative position of two people under this circumstance?
The design of the theater is very special because its round and almost identical to see it side to side.

The point is if the background is identical, you will have to understand their relative positions from a limited perceptive.
Therefore, the camera hides Mumei intentionally. Although it seems like he is disappeared to us because we are deceived by the perceptive from the camera. He is always there.

And finally, all these strange storyboard cuts don't make non sense now. It suggests Nero is neither talking to the air nor Hakuno before the fight.
The person she is talking to is actually Mumei.
I think SHAFT wants to hide this because Mumei has connections to Nero. The relationship between the characters is getting complex again.
Nero said "It's you." before she fights Mumei, if this word is really said to Mumei, it may suggest they know each other because Nero sounds like she is greeting an old friend.