Planetina is a superhero focused on protecting the environment. She and Morty fell in love (don't worry that he is 14 and she looks like an adult, she was artificially created from some magic bullshit so the concept of age doesn't apply to her, so it's totally okay, haha), and he freed her from the evil ex-superheroes who created her and were parading her around as a product. So they got together and it was genuinely very sweet. They went around saving the planet together… until her methods became more and more questionable. When she outright started murdering innocent miners for doing their jobs, Morty was horrified and had to brake up with her.

Obviously, he was right, and what she did was terrible, but you kind of can't help but feel sorry for her. Which is probably why /co/ has been waifuing her hard for the past week.