When you boil it all down, one can see Corak as the true hero of the Sheltem arc, and the heroes we play as are just random adventurers who just happen to walk in on the plot.

Sure you have to rescue Corak twice, but in the end, destroying Sheltem is his quest, and he's the one who succeeds.

At the end of V I was expecting a big epic battle to end all battles with my party against Sheltem. Until the big epic battle was reserved for a cutscene between Corak and Sheltem. At the time I thought it was a complete copout but then that's when I realized, Corak was the main hero this whole time, we're just "assistants"

But then again, that begs the question all the villains I killed, all the people I saved, while Corak is meanwhile either indisposed or focused completely on persuing Sheltem. Who is the real hero indeed?

True heroes are those, who are being controlled by the player :))
It applies not only to MM but to other games as well.

(I was talking through a story perspective)

But yeah, that's true.

Even in something like Undertale's genocide route?

Yes. Even in extreme cases, anti-hero is still a hero, just in a special way.

Aren't you mixing up hero with protagonist.

Aren't they the same thing?

"Looks up protagonist in the dictionary to compare"

~The leading character, hero or heroine of a drama or other literary work.

For me, hero is someone that saves the world. Protagonist is just the main character that a story follows. A protagonist can also be a villain.

Oh true :)

I mean, a slice-of-life never really has "heroes" but protagonists that one roots for. I see what you mean.

There is the eponymous game "Heroes of Might and Magic". It features player-controlled heroes and enemy heroes, who are definitely bad and villainous. It is a good example of heroes as antagonists.

Also some games use word "Hero" as title for monsters. In example, Dungeon Siege II and its infamous Undead Azunite Archer Hero.

This.