If you examine the dynamic between Eren & Reiner, you’ll notice these two are on the opposite ends of the Hero Villain archetypal spectrum (at least to me).
Originally the show makes it seem that Eren (the Protagonist) is the Hero. And Reiner (the Antagonist; Eren’s primary rival) is the Villain. But in Season 4 we begin to see light shed more truth and in reality, Reiner fits the archetypal Hero (The Chosen One selected to defeat the Dark Lord) role much more, and Eren the archetypal Villain (The Dark Lord hell bent on world destruction). Perhaps their name meanings (Reiner Braun; Dark Warrior), and (Eren Jaeger; Hunter-Saint) can also shed meaning? Lastly, there is their physical appearances and Titan forms. Reiner looks much more like your traditional all American hero while Eren appears much more like a social outcast who society ignores. Reiner’s Titan form also seems much more heroic while Eren’s more villainous.
If AoT was made in the west, and was a western fantasy Novel and not a Japanese Manga, I often feel like it would’ve been from the “Hero”’Reiner’s perspective as he and his “fellowship” (Zeke, Berthold, Annie, Pieck, Porco) venture to Paradise Island to defeat the ‘Devil of Paradise’ and his “Demons”, and rescue the princess (probably Historia). Very cliche but this is how many fantasy novels play out. A brave hero must defeat the evil dark lord. But Hajime Isayama turned this trope upside down and showed us Eren’s perspective and painted Reiner in a more villainous light, exposing the flaws in traditional Hero Archetype while giving immense sympathy for the overlooked Dark Lord. And for that I’m grateful. As I’ve always hated the chosen hero trope and felt bad for the dark lords who we never learnt why were evil.
There is also the Helios and Devil thing but I think that’s more a metaphor than anything serious. These two are human beings, not legendary mythical creatures. This post isn’t trying to demean either character. Eren is my favorite character and Reiner my third, right after Mikasa.