i agree, their relationship is just adorable. 
Zadkiel would never claim to love one of his siblings more than the others. However, there was no denying the fact that there was a special relationship between he and Nox. It wasn’t really that he needed Nox to survive, because he could easily make his way through every day alone if he had to. No, it was more that Nox needed him. And quite frankly, Zadkiel loved the fact that she needed him, he loved the fact that she relied on him. He liked supporting her and helping her through the day. It just made him feel good.
Zadkiel knew that Nox’s development was a bit slower than normal, but he saw how hard she tried to excel despite her disadvantages. He watched her through bright eyes as her breathing slowed and her brows furrowed in concentration to form the words she so desperately wanted to come out. Sure, they weren’t perfect when they left her lips, but she had tried, and that was all that mattered.
"Noxxy, when’d you get so smart!" he complimented, referring to her final words. "You’re absolutely right, they could be just like the bears! Y’know what that is? That’s called hibernation," he said, eager to provide her with a new piece of knowledge.
Bending down, he hunched over slightly and released her hand. "Here, Noxxy, I’ll give ya a piggy-back ride and we can go look for those fireflies!" Holding out his arms to support her, he looked over his shoulder, waiting for his sister to hop on for the ride.