Can I request some Invincible characters with a GN S/O whose an eldritch/cosmic horror? They actually do a pretty good job of pretending to be human... as long as you don't stare to long at them (else you'll see them laughing with their eyes). If you've ever seen The Summer Hikaru Died, their true self looks at lot like "Hikaru's" impurity form!
oh this is a fun idea!
Okay, let's see...
Mark Grayson: he probably meets you in space. You notice him first and become interested in this colourful earth boy and decide to follow him. Mark probably notices some "colourful blobby" thing following him, but doesn't connect it to you later on when you first introduce your new human self to him. He probably clicks on real late into the game that you're not what you seem to be, y'know, after noticing you don't get hurt or eat or even breathe... or blink. Or that your eyes reflect light in photos. Yeah...
He's actually really happy you're not a normal person because it means he won't have to worry about losing you as much. What will he have after five hundred years? You.
Conquest: Viltrumites aren't supposed to breed with "lesser" species, so your human disguise really works in his favour. Though it starts to fall apart when he realises you're a little too durable/powerful for a human. And he knows you're not a Viltrumite, so... what are you?
Upon revealing your true form to him, Conquest immediately asks: "Can I fight you?" Followed promptly by: "Can we mate in this form?" Y'know, the important questions. He finds you a little weird looking at first but quickly adjusts. For a universe full of millions of aliens, you're easily the most unique lifeform he's come across after all.
He especially loves how durable you are. Finally, a companion! And one that won't die easily or soon!
Cecil Stedman: ... there's no way this man doesn't know. Someway, somehow, he learns the truth. He practically lives at the GDA which is chock-full of different types of security sensors. One of them is bound to pick up on your inhuman status. And when it does...
He doesn't do anything. Not at first. It's the same as with Nolan: he waits and sees what you do. So long as you're peaceful/helping save lives, he lets you be. And since all you're doing is making him meals and keeping his space tidy, Cecil is happy to let you be.
When the day comes that he asks to see your true form, Cecil lets out an appropriate "... Huh." at the sight. He's not sure what he was expecting, but this sure isn't it.
Then he continues on like nothing happened. A few tentacles/dozens of eyes/void-like body won't stop him from dating you.
Rex Splode: he mentions once that magic freaks him out, so I imagine supernatural stuff does too. So while he likes you, loves you even, he can't help but side-eye you whenever you do weird shit. His magic senses are going off whenever you appear behind him without making a sound or when your shadow remains a shapeless pool under you no matter where you are or what you're doing.
"Love you babe but that's creepy" becomes his catchphrase at some point because you're always doing weird shit. He feels so vindicated when you reveal the truth to him. Like finally!
He likes poking your true form, finds it funny to touch.
Thragg: the phrase "there's always a bigger fish" is true here. Thragg is the epitome of Viltrumite strength, a Viltrumite among Viltrumites, yet you...
You're something else. Something Other.
Endless, eternal. Form shifting and changing on a whim, only glimpses of your true form allowed, else he may become... overwhelmed.
For a good chunk of time he's probably disgusted by you before, after a long time of being with him, he adjusts to your presence. He sees you as a pet for a bit, then a companion. If you're capable of having children and give him an heir, Viltrumite in all but the added abilities of the unfathomable, he'll proudly call you his mate. A strong, healthy child is all he really wants with their species on the brink of extinction, after all.
The Maulers: you pretend for a bit, but the boys aren't stupid. Sooner or later they catch on that you're not human, and not in the way like they or other villains aren't. But as in literally out of this world.
They probably experiment with you for a bit, try and figure you out. Is dating a eldritch entity shocking? Yes. Is your true form headache inducing? Quite. Do they really care? Not as long as they can run tests on you. They are scientists, after all.