There are SO many ways humans and Eridians compliment each other, and I don't even think some were intentional. For example: Lots of fan content I've seen has been kind of thoughtlessly projecting humanity's heat seeking behaviour onto Eridians. Reading the biology document, however, has made me infer something — Eridians love the cold.
They sleep at night, even in the absence of a light cycle, because that's when the temperature drops. Unlike humans, they don't use blankets. They don't try to preserve heat. They want to cool down faster, because because that's what their body needs shut down and rest! They RELAX in the cold!
Not only that, but their version of breathing is a constant influx of cool air. If they're without that coolant for even a few minutes, they're in serious trouble, and after half an hour, they're dead. Unlike us, they're at constant risk of overheating.
But not while they have a source of cold nearby. Like, for example, a human who lives at a fraction of their temperature.
On the Hail Mary, Grace spends a lot of time pressed against Rocky. Their temperature differences are extreme enough that xenonite can't entirely halt the heat transfer, so it's pleasantly warm. He feels like a cat curling up by the fire. It's relaxing. Of course, he also feels a little bad that he's draping his freezing raincloud of a body all over Rocky, but he figures Rocky puts up with the unpleasant cold okay.
Except: Rocky is chasing the cold just as much as Grace is chasing the heat.
Grace's muscles unwind with a constant heat press. Rocky relaxes with en endless source of peaceful, relieving cooling. What a funny quirk of biology, Rocky thinks, that being so incomprehensibly opposite to each other is so pleasant. What a lovely quirk of fate, that they've been given the opportunity to learn that through each other.
