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 campaign1.
The 3rd approach of this answer 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
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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:
The PC is going to be the pre-created rogue of the Starter Set story and the GMPC is going to be the Mage with his book of life story as named bond.
I need to characterize a trait, behavior or background for the GMPC 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 or background — when role played correctly — prevents him from initiating decisions, or otherwise participation in decision-making,2 while still having a valid motivation for the success of this two person party?
Footnotes:
1 Among the cons of a starter pack, as I assume most know, is its 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 exceptional case not caused by the GMPC's player himself.
Best Answer
Make a GMPC that cannot help
Good examples of this are for example golems, zombies, or animal companions. You can ask them for help all you want; they cannot make decisions for themselves and only follow orders. They might instinctively fight on their own accord, protect their charge with their lives and follow him anywhere but no matter how often you try, they won't be able to pick whether you should go rescue the princess immediately or find the magic sword first.
Make a GMPC that is not allowed to help
Perhaps the companion is tasked by a Holy Order to escort the PC, because according to a prophecy the PC will recover an ancient artifact. But the prophecy also says that the PC will travel there on their own accord and their minds cannot be influenced by those who know the contents of the prophecy. So the companion will guard and help the PC with anything they ask, but they cannot influence their decisions directly for fear of ruining the prophecy.
Alternatively, the companion could just have a very strong belief that the PC should be solving this problem on their own. They could be a mentor from a monastic order teaching them about decisions and responsibilities, for example. Or a military leader training an officer to deal with complex situations.
Make a GMPC diametrically opposed to going forward
For example, a bodyguard who is under strict orders to follow the PC wherever they go and never go against their command, but who really just wants the PC to go back home and give up this foolishness. They will never say so, but if you ask, their only advice is "turn around and march home", which is not exactly helpful.
Make a GMPC that cannot communicate with the PC
A mute, for example. It would take quite some adventuring before the PC can learn sign language to the point where the companion can do much more then point at things and do some gestures. This one can give some help, but as long as you as the DM don't talk it's very easy to stay in character and you probably won't be able to give away much. (It's also funny to have to do charades)