I'm finally after a long time of thinking about it, going to play a campaign of DnD. For that I have bought the starter set and I'm actually still in preperation for it. I'm going to be the DM and it is going to be a 1 on 1 campaign¹.

I found the 3rd approach of this answer:

http://rpg.stackexchange.com/a/59433/15211

is a way of doing it without too much effort while it still leaves the chance for 2 first timers (as we are) to concentrate on roleplaying just a single character.

I'm planing to play 1 character so my friend can play 1 aswell. In addition I'm planing to just decrease the monster count so, that the XP treshold comes closest to the equal dificulty-class for just 2 players. For bosses I bought the monsters-manual, so I can look for an boss like monster that still fits in the lore with equally fitting difficulty.

But there is one point left in my preperation that is hard for me to find a way to solve.

I know my self well enough, to know: When I will be DM'ing a "story" that I allready know, while I'm playing a character that is part of it, as long the character itself has a motivation to get ahead, I'll be playing it in an impatient pushing way, since I'm not good enough to winging the behavior of the character for every situation. I could make a plan when I have to respond in what way, but... all I learned so far, this won't help either, because thinks are going to come unexpected.

But I wan't to present the roleplaying aspect to my companion aswell and I guess it will be a less intense expierience, if there is no PC to interact with.

So I got so far:

I need to characterize a trait, behavior or background for the character that it has no ambitition to push the story on. (so the PC can't become a tool of me to bring the story where I passively wan't it to get to and still leave the unfolding of the story in my companions hand.)

But all ideas I had so far about an character that would satisfy this need, had one big flaw:

When I asked my self "And what is his motivation to even participate in this adventure?" There was no feasible answer to it.

So:

TL&DR

How to design a character in a way, that his trait, behavior or background when roleplayed correctly lets him not participate in decision-making², while it still is obvisious that his motivations are a succes of the "party"?

¹_{A cons of the starterpack is as I assume most know, it's difficult to play in this given set up.}

²_{Should the character be forced by another PC into making a decision, it is acceptable, since it is an exceotpional case not caused by the PC's player him self.}