the Main Character is *per defenition* important to the story. Even if that character would not exist if not chosen as a Main Character.
This does not make any sense. How can a character that may not even exist in the story be important to the story? If a story can be told without a particular character's existence, then that character is not important, by definition.
This is why 'Tav' is NOT at all the same as 'Charname' because although Charname also could be defined by the player in whetever way they wanted, Charname as a person always exists in every playthrough. But Tav may or may not exist in the game world. That makes Tav expendable and superfluous to the game's story.
Because every story has a main character.
The Story where you chose Tav is a different story than when you would not chose Tav.
Let us say you chose Wyll as an Origin character. Tav does not exist. Thus Wyll is the Main Character and has the character development of a Main Character. He is no longer the "Wyll" that was defined by his past. He has new options for development.
Thus the story is fundamentally different than when you would chose Tav.
Because in that case, Tav is an influence on Wyll, but Wyll is still bound by his past.
Tav does not exist if you do not chose that "origin", true, bis the *lack* of Tavs existent has an effect on the story.
This can be seen even more with Dark Urge. That Character also does not exist if not chosen as a Main Character. But DU can also do things, achieve things, no companion can. Again, the lack of DUs existence changes the story.
Can we have a Story without Tav or DU? Yes, obviously. Will it be the same story as with either of them? Definitely not.