The relationship between the Hulk and Bruce is one likely the most complex Tony’s ever encountered. Gamma radiation created the Hulk, that much he knows, not from some outside source, but from Bruce himself. They talk about each other like they’re different people, and Tony talks to them both about each other like he’s in complete agreement. And they are distinct personalities; the way they speak and interact with the world, it’s clear that they’re not the same. But the Hulk is still part of Bruce. He’s a big green part of him, made of anger and, he’s learning, loneliness, but he’s still an aspect of Bruce.

And Tony cannot abide any part of Bruce being lonely. Not when he’s there to prevent it.

“I promise.” He can’t remember when he’s last sounded this serious to his own ears. “Cross my heart. Both of them. I’m not going back on that promise. Not ever. You and me, we’re in this mess together. For keeps.”

In the kitchen of some random house in some world that wasn’t his own, Tony did his level best to drive home a point to Bruce about monsters. He remembers it clearly. Just like he remembers what he said, only half joking. Well, the Hulk is close enough now, and it’s not just with Bruce that he feels he needs to make a point. So while he’s there, before the Hulk moves, Tony leans in just that last little bit, and brushes a kiss over his lips.

“Thank you doesn’t even begin to cut it, but for everything you’ve done since the moment that experiment happened, I thank you. And I got your back against the whole damn world if it comes down to it. I promise you that, too. Okay?”