The sight of Gamora through the portal didn't affect her. It was like watching something on a visiscreen. It didn't mean it was real. She couldn't even trust that the actions she was seeing were real or illusion—like how apparently willing and eager Gamora was to come to her.
Then Gamora stepped through the portal and was in front of her. And suddenly, that was real.
Again, Nebula stood very, very still.
Then, to no one's surprise greater than her own, she'd lunged forward and grabbed Gamora—