Simon's mutations mean Erid's gravity has no impact on him — and they don't know it, but he can actually survive fine in the atmosphere, too.
Meanwhile, the starvation and loss of muscle mass means Grace is even more susceptible to Erid's intense gravity. By the time they meet, Grace is already using a wheelchair to move around.
Simon's there four years before Grace's body gives up. The Eridians have no choice but to place him in stasis until they find a solution.
Simon's life the last four years has been a beautiful dream. For the first time in a long time, he's safe. He has people — well, one person and multiple aliens — who care about him. They find him funny! They seek out his company!
There's always food. When Grace finds out he's not from Earth and has never had cake or spaghetti or french fries, Grace moves heaven and Erid to make those foods for him. Simon cries the first time he tastes strawberry shortcake.
Simon gets to see trees and plants and can pick fruits and vegetables from the greenhouse.
On his dark days, though, he feels undeserving. Grace and the Eridians have given him everything, and he can give them nothing in return.
He can fix things, but he's no engineer. He's not able to teach the Eridians science they haven't discovered and change their society. He's just a freeloader.
But then Grace is put in stasis, and Adrian wonders aloud why it is that Simon isn't impacted by Erid's gravity. They ask for a sample of his blood, and Simon doesn't hesitate — he'll bleed himself dry to see Grace smile again.
And they discover a lot of things. How Simon was made less human by The Blood Ocean, but how it's this terrible thing that happened to him that will save Grace, that will allow him to thrive in Erid the way Simon has.
He's been treated less than human his whole life, and then he finds a place where he's respected and valued, and it's there that he finds that he's now the Less Than Human he's always been treated as.
Except no one even CARES how human he is or isn't, and they all love him just the same, and also he SAVED Grace, and Grace wakes up and smiles still a bit tiredly, and he's THANKFUL for The Blood Ocean, and that's a whole 'nother can of worms, and this whole thing has basically meant having about ten cans of worms he needs to deal with, except Grace is SAFE and that supersedes everything else, and — shit, is he in love with Grace?