In those moments, when the darkness is silent except for the study whir of the idling engine, he'll whisper her name softly.
"Kitty?"
And he'll look over and see her sleeping form, cuddled compactly, body tilted just to the side, fists clasped against her chest like she was about to get into a fist fight or shake maracas, head resting against the pillow with her face tilted slightly towards him, breath slow and rhythmic from her open mouth, chest rising and falling in time with his heart.
It's in those moments that he will speak things he never says to anyone. In that silence that he tells her all the things he hates about himself, all the terrible acts he's committed, all the people he's hurt and killed, all the times he's acted like an asshole to everyone in some feigned attempt to protect himself.
He tells her about his mother, and the day she died. He tells her about the orphanages, and Yondu, and space. About mixed memories and the Cancerverse and seeing Richard for the last time. He tells her about Ship. He tells her he doesn't know what kind of person he is, he doesn't know who he's supposed to be or how he's supposed to act. He tells her he doesn't want to be the President of Spartax, or even royalty.
Sometimes he cries as he tells her. Silent tears slip down his cheeks, and he wipes them away with the bed sheet. He doesn't sniffle or whimper, he just accepts his moment of weakness in silence, in the cocoon of darkness. It is safe, he will not be seen, he will not be mocked.
He curls into her, pulling her angry little fists to his chest. When, as she sleeps, her fingers relax against his skin, he feels a tiny, uncontrollable joy, and he realizes he's feeling real, true love for the first time in a long time. He wants to trap that feeling and keep it with him forever, but he knows he can't.
He knows how fleeting happiness is, so he commits every moment, every micro movement, to memory. He holds her close and breathes in her soft warmth. Before he drops to sleep, he speaks one final time.
Every night, it ends the same way, he says the same words, he quietly tells her, "I don't think I'm good enough."
He never specifies for what.