I like the "simon calls grace angel" headcanon, but to me its like this:
Simon doesn't believe in what the Father preached. Not anymore, at least. The last time he fully supported it was before he became the convict. You don't last in a prison like that wondering when a god will come and save you. You pick yourself up by your bootstraps and save yourself.
When he meets Grace for the first time, hes afraid. He's been afraid a lot recently, what with the blood ocean and Ellie. And yet, when he sees him- his saving Grace- he thinks "Father was right about the Angels." Then he passes out.
When he wakes up, finally in his right mind, he doesn't start calling Grace an angel immediately. He doesn't want to push his boundaries and plus no normal person wants to be put on a pedestal like that. But Grace is so nice, and it's so confusing. No one back home would be so willing to share their resources like this. No one would be this selfless.
He stays silent- or at least quiet- more often than not. Grace attempts to get him to talk, but Simon doesn't want to do anything that could make Grace angry. He doesn't want to die. He keeps to himself, stays out of Grace's way, and when Rocky tries to talk to him, he avoids them too. After all, Rocky was Grace's best friend. If they wanted him gone, it'd happen.
On a day where Simon's scars ache and his mind fills with redredred he snaps. Grace was trying to get him to talk again, and he felt backed into a corner- like a wild animal. Like a kicked dog.
"You must think you're some kind of angel, huh? Think you're so benevolent? Prodding at me like I'm some kind of passion project?"
And. Well. Grace might be kind, and too selfless, but he isn't a push over and he definitely won't stand for that. He bites back, saying something like "What is your issue?!" or "I was just trying to help!", and then they're fighting.
Grace fights dirty- dirty like how Simon's brothers would fight over leftovers back on Eden after being punished. Dirty in the way where it couldn't be taught- it had to be earned. Dirty in the way where it was familiar. Simon was down an arm he still wasn't used to being without, but Grace was weak with early malnutrition. In the end they tied, panting and slumped on the ground.
After that they seemed to understand each other more. Simon saw Grace as an equal, rather than the man who was keeping him around and could remove his resources anytime he wanted. Grace saw Simon as what he was: a scared man who wanted a place to belong.
They still butt heads sometimes, but eventually the "Angel"s that were dipped in pettiness turn into something softer.
"Angel, come look at this."
"Angel, what's this mean again?"
and when they get to Erid (exhausted, on the brink of death, so so lucky to have survived) it only softens more.
"Grace, Angel, the kids will be fine if you skip a day."
"Rocky and Adrian are here, Angel. Time to get up."
Neither of them noticed when the change happened. Neither of them cared. When they laid together at night, limbs pressed together and warmth seeping into the sheets, they couldn't think about anything else. Maybe they didn't have to. After all, Angels weren't real, but if Grace was the closest to one, then it'd have to do.
If man was made in God's image, whose to say angels weren't made in man's?
