In book 3 (The Hidden Kingdom), page 163, when Tsunami and Glory found Deathbringer coming out of the tunnel.

"Sorry, I couldn't read your mind like a stupid NightWing," Glory said. "Speaking of which, are you the mind-reading kind of NightWing?" She narrowed her eyes at Deathbringer. "What am I thinking right now?"

"That I'm much too charming, clever, and good-looking to kill?" He guessed.

That was actually a little closer to accurate than Glory liked. 

In book 4 (The Dark Secret), page 267, when Starflight and Glory are saving Deathbringer and Splendor.

Starflight glanced around and realized that Glory ad gone right past them. She was at Deathbringer's cage, grabbing the bars. As he watched, Deathbringer reached his talons through the bars and wrapped them around hers, and they exchanged a look that said "thank you" and a whole lot more.

In book 5 (The Brightest Night), epilogue, when they are talking about Queen Scarlet.

"Fine by me," Glory said, and despite their sarcastic words, the look they gave each other made Tsunami roll her eyes at Sunny. 

'I wonder if they'll have dragonets one day,' Sunny thought. 'Would anyone complain that the Queen of the RainWings should be with another RainWing instead?'

In book 7 (Winter Turning), page 66, after Winter, Moon, Kinkajou, and Qibli find the body of the NightWing Icicle killed.

"Don't get your tail in a knot," Glory said to Deathbringer. Her sloth was already clambering up her tail onto her back again. "My top secret invisible guard was following us too, and you know it."

"But I wasn't," He protested.

She gave him an exasperated look. "You can't always be," She said in a low, surprisingly affectionate voice that suddenly made Winter rethink everything he had assumed about the Queen and her bodyguard.

 In book 8 (Escaping Peril), page 255, when Moon, Qibli, Turtle, Winter, and Peril are looking for Chameleon.

"I AM," Peril said. She thought 'MUSHROOMS AND MONGOOSES' at the NightWing as loud as she could, and Moon started giggling.

"That seems like a good sign," said Qibli, giving Moon a smile that reminded Peril of that NightWing who always followed Queen Glory around.

I know this is more like Moonbli than Glorybringer, but still.

SO THERE'S PROOF. I COULD GO ON, BUT I WON'T. THESE ARE FROM BOOKS 3, 4, 5, 7, AND 8. IF THIS ISN'T ENOUGH, I DON'T KNOW WHAT IS.