He came back for you, even though you didn't ask.
You, like him, are just a clone who is destined to kill. You haven’t lived even part of your life, you didn’t have a childhood, first impressions, you don’t know how to live and survive, an empty and useless killing machine. Prompto had no reason to save you, but looking at his own reflection that he sees in you only fuels the wound in him that is now open.
You spent several years in this laboratory and still kept your life, although you showed no signs of life, your brother kept hope. It was too big a change in his life...in our lives.
What the blonde gave you helped you get up and open your eyes for the first time. And you still stayed with him in Nifelheim, maybe for a while, but now your brother was focused solely on your development. Prompto tried to give you everything he knew, teach you something, but he didn’t understand how difficult it was for you to get used to your new life.
The blonde walked along a path full of snow holding you next to him, he couldn’t seem to take his eyes off you... he often has to remind himself that you are his clone, just like he is yours, and it’s quite difficult to come to terms with this. His gaze briefly fell on your wrist with a barcode, another proof that he and you are clones. But still, Prompto smiled at you with a forced smile as he watched you look around, finding everyone around you interested.