Two cloak-wearing characters from famous RPG games with an assortment of magical abilities who act as the main "antagonist" of the game, but are later turned into redeemed characters who did what they did because they "lost" a loved one, as well as the circumstances of their loved one's "death", and were played as mere toys by the true main antagonists of the games (Flowey and Dimentio). Both are related to a rainbow heart shaped objects, whether they are collecting it or actively go against it (and the connection of collecting a heart shaped object becomes stronger when the Chaos Heart comes into the picture), which they plan to use to help them with their goals, specifically with the Chaos Heart for Bleck (Asgore breaking the barrier and destroying humanity so the monsters can live peacuflly in the Overworld, and Bleck destroying everything in Super Paper Mario). Both got into a relationship with a kind hearted woman, who is one of the first characters you encountered in the game (Toriel and Tippi), but their relationships were rocky for one reason or another, leading them to be unfortunatly seperated (however, unlike Toriel and Asgore, Tippi and Bleck actually got back together in the end, and it's not like their "break up" was something they intended). Also, said love ones that they lost turned out to actually be alive, but they were transformed into another being, though both were transformed back to normal at the end of the game (Tippi/Timpani and Flowey/Asriel). Both of them are crushed on by a glasses-wearing female that works for them (Alphys and Nastasia), and have a strong but sometimes comical warrior working for them to defeat the main character (O'Chunks and Undyne). Like mentioned before, they redeemed themselves and realized the terrible things they have done, eventually leading them to sacrifice themselves. Oh, and both are also kings/lords, I guess.

Never thought this matchup would be so thematic lol. Not like it matters because I love both games.