His mind is bright, sharp, clear, the loudest one on Serenity and she knows she wouldn't quite be able to shut him out, even if she wanted to. She can feel him now in the engine room working on modulating the power outflow, Midbook Transport. Standard radio and accelerator core. Class code 03-K64—Firefly even though Kaylee locked him out at least five times; he keeps finding his way in. River always has to lie down when he starts working, equations and simulations running through his brain fast as lighting, louder and louder until his thoughts are screaming at her deafening enough to make her brain dizzy. It's getting very, very crowded. She's taken to lying in her bunk when she feels him start up, letting his thoughts wash over and through and around her. He fascinates her; ever sense River first heard his thoughts, panicked and confused and angry, on Persephone, she's been drawn to him like a magnet; opposite ends pulled toward each other as he rappels everyone else away.
She winces as she hears the ship cry in fear, scared of these unfamiliar hands and what they're doing. River comforts her, murmuring like she feels Kaylee do when Serenity needs a little boost. That's my girl, that's my good girl.
She tried to comfort him at first, trying to explain how she felt when she woke up for the first time and found herself not in the home she was expecting. No, no, we can't go home. If we go home, they'll just send you back to the Academy. But she can't get the words right and black chaos thick as oil spills from her lips instead and he looks at her like she's not quite human. No one can shoot like that that's a person.
But still, River lets her mind wander through his sometimes. She loves running her hands over the Stargate, stepping back as the event horizon forms, and letting wormhole theory trickle through her brain. The city speaks to her as it spoke to him; Atlantis tells her its secrets, shares its pain, lets her taste the feel of Ancient technology under her hands. She learns to fear the reavers-that-aren't-quite-reavers as much as he does, and even starts to shiver whenever someone places a hand upon her chest. There's too much screaming. Simon changes her meds, convinced this new aversion is a reaction to something she's taking.
He wants to find the city so bad the thought exudes from him, bleeding into every thought that enters River's mind. She's found that she's started to look for the city too; wandering around the ship looking under pillows, rifling through Inara's things. She's just a troubled girl. He dreams at night, and therefore River sees Atlantis laid out before her, and cries out when the floor falls away, and spends the rest of the night chasing after it, trying to get back—to get home.
He associates home with stain-glass windows, a dim hum in the back of his head, but most of all with a slow, easy smile, a mop of wispy hair, and a Scottish brogue. People whom he saves—who save him. When you can't run you crawl, and when you can't do that... Mal can't decide if he's worth the annoyance; Jayne's mad that they've taken on another useless passenger; Kaylee tries to comfort him Isn't there anything about this place you're glad of?; Book tries unsuccessfully to explain the importance of faith; and River...
River knows that like she did, he will learn to call Serenity home.
Once you've been in Serenity you never leave... you just learn to live there.