Okay so in the PHM book Grace mentions that, since his last name also happens to be an English word, Rocky just calls him the Eridian word for “Grace”. But considering “grace” is a relatively niche word that would probably take a while for them even to develop the vocabulary to translate, I can’t help but wonder if there’s a little more going on here, and have ended up with two conflicting headcanons to explain it:
When they first meet, Rocky is just as worried about whether the alien will be hostile, and is so relieved by their first interactions being so positive that he names the alien the first thing he has discovered about him: “Kindness”. (It isn’t until Grace is explaining the possible interpretations of his name much later that they realize the coincidence. It feels strangely fated.)
Rocky has been calling him “Idiot [affectionate]” this entire time and lied about the whole “grace” thing when asked because who’s gonna know the difference. (when they make it back to Erid, Rocky has to make some very hasty, awkward explanations to the leaders planning to welcome the second savior of their world, culminating in a heated debate over whether to change the human hero’s Eridian “name” and in doing so expose Rocky’s crimes, or go with a planet-wide “commit to the bit” and keep the “Welcome to Erid, Idiot” decorations as is—all while Adrian loses their mind laughing in the background)