Gil had some difficulty referring to Eden as his brother. In the beginning, when he had met him, he was sure that Eden was Kye, and in fact, he was right because it was the same body and Kye's mind was still there, but Eden was still someone totally different from his twin.
And deep down, he knew that, so he couldn't refer to Eden as his brother yet because it would just be strange.
However, since he seems really genuine about what he was saying and he worried about him like his twin, so he decided to try to call him 'brother'.
It won't kill him to try, after all, so he tried. And after saying it, he wasn't disturbed or anything. It was still as natural as always as if Eden was veritably Kye.
"I'm glad then," Eden said with a big smile. "Anyway, it's time to sleep bro, we still have classes tomorrow."
"Yes, you're right. Sorry for everything."
"Why are you? You didn't do anything."