When I looked at the test print, I thought the tone of the whole background including Sechs, except the skull robot's head in the front, should be a little lighter, so adjusted the area by applying the adjustment layer on it.
Adjusting images with the adjustment layer is convenient for the final adjustment, because you can change later without limit if you don't like it.
Select "Layer" menu -> "New Adjustment Layer" -> "Tone Curve."
I made the middle tone a little lighter.