[Zero does know it's for the better. Himself aside, it's far more stressful and worrisome for Ren. It isn't the way he wants to start their relationship here.
He smiles to that. Something probably more cute (for lack of a better term) than maybe Ren, or anyone, has really seen. While all at once also so fond.]
To be called that. . . it feels amazing.
[For so much of his life so far, he's honestly never thought he would have a family of his own. While for a good part of that, he hardly felt deserving of something that would end up to be so wonderful.