Fluff because rain :) Also as sad as it is, I dig the headcanon of Keith being orphaned as a wee babe
"Listen."
It's the familiar, warm gentleness that Keith speaks with that quiets Shiro. They're taking shelter from the storm under the black lion, watching the rain come down in moving sheets with the wind. Said lion is huddled with Keith's red, and Shiro can tell it feels safe and relaxed by the way Keith is acting.
"Impressive," Shiro says, nodding towards the open air.
"It sounds," Keith says, leaning against a lion's paw - Shiro can see the recent stress and anxiety melting off him. "Like rain on an old tin roof." Shiro notices that Keith watches the rain with a sedated expression, eyes heavy and stance laid back. The wind muses his hair as it pleases and the lightning dances in his eyes, and Shiro comes to stand by him. They watch the rain come down and the lightning chase across the sky, feeling time standing still.
"One of my foster families lived in this beat up old place in the middle of nowhere." Shiro looks to Keith wordlessly, watching the purple in his eyes flash brightly. "I hated it. It was always too hot or too cold, cramped, and there was nowhere to go. Except… Except when it rained. There would be these big, gray and black clouds that would roll in over the mountains - out of nowhere. And it would rain and rain.
"And I remember at night, I would lay in bed for hours and just listen to the storm. The wind howling and the trees rustling - and the rain. The rain sounded so… So good when it would come down on their roof. I can't - I know it sounds weird but… I can't describe it. It was my favorite thing in the world."
Shiro notes Keith's stare - his attention has gone completely from this realm. Shiro wraps an arm around Keith's waist, and Keith responds by leaning against him instead, head on Shiro's shoulder. He's calm and warm, and smells like home.
It's then that Shiro realizes there isn't anywhere else in the universe he'd rather be right now - not on the castle ship, not on Earth, but there with Keith under the rain as he glimpses pieces of who Keith really is. And he says to Keith, "When this is all over and we can finally stop fighting, we'll find a place to settle. Somewhere rainy, and we'll have a house with a tin roof so we can lay in bed together and listen to the rain fall on it."
There's a moment of quiet, where Keith smiles and closes his eyes and responds with a quiet, "I think I'd like that."
