I found a way to make this ship I came up with conceivable! [Context] Their relationship is not an ideal one at all, by the way.
CW: Abusive relationship • manipulative behavior • unhealthy character dynamics • one-sided • no comfort •
The AU takes place in a reality where Shadow Weaver died right after she was cut off from the Black Garnet. And Catra is a biromantic lesbian.
Rick finds himself stranded on Etheria, No, he can't just portal out of this adventure. His portal fluid is utterly drained, and the necessary ingredients for interdimensional travel are harder to come by here.
He joins the Horde so he can get his hands on their tech/resources. Also, in his opinion, they're less annoying than the magic-wielding crowd. There Rick meets Catra, whom he manipulates into seeing him as a "real friend: the only one that won't ever hold you back, unlike the Scorpion chick who's trying to get into your pants." Unfortunately, she is none the wiser.
They're both good manipulators. However, Rick has been at it much, much longer than her. In addition, he distances himself emotionally from others so hard, his tragic backstory had to be taken out of his head for us and Morty to learn the truth. Rick wouldn't have revealed that info himself under normal circumstances. Catra pushes people away as well, but she takes more emotional risks than him. E.g., Light Spinner.
Speaking of taking a risk, Catra falls in love with Rick and spills the beans. He "returns her feelings."
From Catra's perspective, the scientist has been a better friend than everyone else in her life: Rick is supportive of her goals – he even helped out by injecting mind-influencing nanobots into Hordak! He's a jerk to others ("heh, he verbally evicirated that jerk I don't like."), but not with her. And, above all, he thinks she's fine as is (again, Catra's POV). She could never satisfy Shadow Weaver, and Adora wanted her to become a goody two-shoes... Rick's not like them.
Actually, Rick considers Catra an annoying, little tryhard who needs to fucking relax. His very first interactions with her resulted in both of them disliking the other, until he decided there was more to gain from manipulating the Horde officer rather than irritating her.
At first, manipulating Catra had amused Rick. She'd been nothing but a useful annoyance to him back then, the little shit. Now, after seeing more of the catgirl's soft and vulnerable side, he has mellowed out somewhat. She's still a means to an end, yet he doesn't mock her internally when she expresses her love for him. Maybe he feels somewhat bad about exploiting that love?