I'm going to play a campaign of D&D using the Starter Set.  I will be the DM, and it is going to be a 1 on 1 campaign<sup>1</sup>.

The 3rd approach of [this answer](http://rpg.stackexchange.com/a/59433/15211) appeals to me as a way to do this without too much effort, while it still leaves the chance for 2 first timers (as we both are) to concentrate on roleplaying one single character each.  *I want to present the roleplaying aspect to my companion*.  I chose this approach since I feel that it will be a less intense experience if there is no PC to interact with

 - I'm planning to play a character so that my friend can play one
   character as well.

  

 - I'm planning to decrease the monster count so that the XP threshold
   comes closest to the equal dificulty-class for just two players.

 

 - For bosses I bought the Monster Manual, so I can look for a boss-like
   monster that still fits in the lore with equally fitting difficulty.

**The hard part, and my problem:**

I know myself well enough to know that when I will be DM'ing a "story" that I already know, while I'm playing a character that is part of it, that as long the character itself has a motivation to get ahead, I would play it in an impatient, pushing way.  I am not good enough at "winging" the behavior of the character for every situation, so I need a design with limits and restrictions on the GMPC.  It's all well and good to have a plan that will respond in a certain way to a situation, but this won't help when things come up unexpectedly.

Where I am now in GMPC development: 

I need to characterize a trait, behavior and background for the character that it has no ambition to push the story ahead.  (*The GMPC must leave the unfolding of the story in my companion's hands* -- my companion will be making decisions and choices.)

As I tried out ideas for this character concept, I discovered a flaw:  

When I asked myself "What is his motivation to even participate in this adventure?" there was no feasible answer to it.

**TL&DR**

How do I design a GMPC so that his trait, behavior and background -- when role played correctly -- prevents him from initiating decisions, or otherwise participation in decision-making,<sup>2</sup> while still having a valid motivation for the success of this two person party? 

<sup>1</sup> Among the cons of a starter pack, as I assume most know, is its difficult to play in this given set up.

<sup>2</sup> Should the character be forced by another PC into making a decision, it is acceptable, since it is an exceptional case not caused by the GMPC's player himself.