If you've played through the main quest of Oblivion, and were paying very close attention, you may have noticed something odd: You aren't the main character.
Oh, sure. You look like the main character. You're the hero, right? But that's not what the main character is. The main character, roughly speaking, is the character who drives the conflict, whose evolution reveals the theme. The character whose qualities define the story.
That's not you. That's Martin.
He's the character with an arc. He's the character who figures out what the theme is. He's the character you have to pay attention to, if you want to know what the theme is. Without him, there's no story; and if he were a different kind of person, the story would be different.
It's actually a really elegant technique. If a game has you controlling the main character, it either has to take some control of that character away from you, or it has to have a really basic story. Shift the thematic focus to an NPC, and you can be whatever wacky person you want, have whatever motivation suits you, and the game can still have a coherent thematic conflict.