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While Nathanaël did go from being considered essentially a main character by most of the fandom to Background Character #20, I'd say Marc is the one whose only purpose for existing is the ship. He didn't have a reason to be introduced outside of being Nath's boyfriend, because like. If we really needed a writer who shared Nath's passion for comic books, Alya was right there.
Hi! It feels weird to see a message in my inbox that isn't spam, haha!
I once read an interview with the person who inspired Marc's character (Hope Morphin, a friend of TA's), where they said that when they started creating Marc, he asked them who their favorite character was, to which they replied it was Nathaniel (by the way, Hope mentioned that TA complained a bit because people liked secondary characters, lol).
With the question, it was already clear that Marc's introduction would be through another character, maybe even a partner or romantic interest, regardless of whoever Hope's favorite was. So yes, Marc exists to be someone's partner, although I don't know if it's just me, but even before Ruler, I think he had more shine as an individual. He likes to write, but he doesn't limit himself to art (Like Nath). He also likes football. He's also had a change in attitude (from extremely shy in Reverser to more confident in Penal Team). Yeah, these are just a few clues about the character, and to be honest, I haven't watched the show in a while, but I only remember him appearing in a duo with Nathaniel, with Marc standing out more (I suppose because of the differences in their designs and personalities), and if I remember correctly, he had more lines.
In the end, they were just there as a secundary couple to set the scene, "plot tools." But now I think that changed a bit with Ruler. Besides, they might have a real influence on the main plot. It's also worth noting that I'm a Cloenath stan, so I prefer to watch them individually, which is why it bothered me a little that their existences were limited to their relationship and art. You know how much I needed a reaction from Nathaniel about not being able to see Alix, his best friend, again?
(By the way, this was translated with Google, and I haven't seen Season 6 yet, so this might be a little weird or misinformed.)