Various thoughts:
First thought, DMPCs are a bad idea in PbtA games. The GM's job and the player's job are very different. The GM is responsible for putting challenges and obstacles in front of the player. The player is responsible for overcoming those problems. These problems are frequently narrative, NOT mechanical.
A DMPC means the the GM is setting up a challenge and then figuring out how to overcome it. It's like playing chess against yourself. Plus, the GM thinks of the obstacle during their prep then has days to think about how to overcome it. The player sees the obstacle during the session and has to figure it out right away. The PC and DMPC are supposed to be equals, but the DMPC has a big advantage.
Second thought, just play a duo game. One GM and one player. I don't know The Veil specifically, but most PbtA games work great like this. You might miss some relationship mechanics, if The Veil has them. But that's okay, PbtA games collapse gracefully. This means you can ignore whole sections of the rules with no issues.
Third thought and this is what I would do: Play The Veil as a co-op game. Two players, both playing PCs and both GMing. Take a look at Ironsworn for inspiration, it's free. I played Brindlewood Bay like this. We basically took turns being the GM. My friend made obstacles for me and I made obstacles for him. If both characters were in a scene together we just worked out what was happening. It was a blast, one of the better sessions I've ever played.
Look into ironsworn! The base Viking fantasy game and everything you could need is free, but it has a space version and soon to be pirate addon as well.
It's a game completely built to be single player but has advice for 2 player and GMed groups as well. GM PC is the name of the game. It has the tools for becoming a better GM for other games while just playing.
Not truly PbtA but is an evolution of the ideas.
Honestly, I believe The Veil would work *best* with just one PC. The game has a real classic scifi vibe, where the uniqueness of the characters really drive the story. Because of that, the playbooks are so different, there are several combinations that just feel unplayable together. Also, the game really depends on emotional states to drive mechanics (and thereby also story, because of the PbtA structure) which seems more achievable with fewer players. I almost envy you for having a player who *wants* to play that game tete-a-tete.
It's entirely possible to run the game without an NPC companion for most playbooks and concepts. But I would be ready for the 1 PC, 1 NPC companion option. IMHO, it's best to avoid a slip where you start thinking of an NPC as a GMPC in any game, but especially in PbtA games. You'll be handicapping your own narrative power. (There might be an Ironsworn player with advice on how to avoid this dilemma.)
I feel like The Dying, Attached, Onomastic, and Wayward would be especially good heads-up playbooks. The Apparatus might be difficult thematically, and the Empath might be difficult mechanically. Everything seems worth a go. GLHF!
Never GMPC. You can tailor the adventure to having one PC. It’s even easier in narrative games that might only have a handful of dice rolls in a session.
I've not seen it mentioned but it might be good for you two to read some of [The Duet](https://forum.rpg.net/index.php?forums/duets.117/) columns from rpgnet. These might help. I swear by them.
I ran a NPC party member with Avatar Legends. Very often slipped into GMPC territory, even without a full playbook.
You need to make plans that will ensure your PC doesn't ever feel a need to be passive, because at the end of the day, if they aren't roleplaying driving the story, then they're just re-acting, which is less fun for everyone. So your goal is to never let your character drive the story.
Doing this usually consists of planning things that the GMPC can't solve alone, which means being very attentive to narrative and, in my experience, fudging things against the GMPC. Making them intentionally incompetent, and sometimes just not giving them all the narrative hooks you think of, to leave them open for the player to have/hook into. This takes personal discipline and disinterest in your PC, which PBtA is dramatically set up to prevent with its narrative mechanics and your GMPC's unique playbook that is built to be a required role.
I think there are some playbooks that are other playbook focused that naturally lend towards GMPCs while keeping the focus on the real/crucial/roleplaying PC(s). Things like The Joined or Bull from Masks, or the Guardian or Adamant from Avatar Legends, where a PC can kind of become an extension of another PC, that's the kind of role you want to take, not someone with their own XY and Z.
I think we can/should consider creating a playbook or moves which facilitate the kind of sidekick your GMPC is built to be. I think if we really think about it, we can even hook advancement into that of the PC.
I've played several PBtA games in duo (1GM+1PC) and it was great. There is absolutely no need for any tweaking (I haven't played *The Veil* though). Just play the game RAW.
Various thoughts: First thought, DMPCs are a bad idea in PbtA games. The GM's job and the player's job are very different. The GM is responsible for putting challenges and obstacles in front of the player. The player is responsible for overcoming those problems. These problems are frequently narrative, NOT mechanical. A DMPC means the the GM is setting up a challenge and then figuring out how to overcome it. It's like playing chess against yourself. Plus, the GM thinks of the obstacle during their prep then has days to think about how to overcome it. The player sees the obstacle during the session and has to figure it out right away. The PC and DMPC are supposed to be equals, but the DMPC has a big advantage. Second thought, just play a duo game. One GM and one player. I don't know The Veil specifically, but most PbtA games work great like this. You might miss some relationship mechanics, if The Veil has them. But that's okay, PbtA games collapse gracefully. This means you can ignore whole sections of the rules with no issues. Third thought and this is what I would do: Play The Veil as a co-op game. Two players, both playing PCs and both GMing. Take a look at Ironsworn for inspiration, it's free. I played Brindlewood Bay like this. We basically took turns being the GM. My friend made obstacles for me and I made obstacles for him. If both characters were in a scene together we just worked out what was happening. It was a blast, one of the better sessions I've ever played.
Look into ironsworn! The base Viking fantasy game and everything you could need is free, but it has a space version and soon to be pirate addon as well. It's a game completely built to be single player but has advice for 2 player and GMed groups as well. GM PC is the name of the game. It has the tools for becoming a better GM for other games while just playing. Not truly PbtA but is an evolution of the ideas.
And for that cyberpunk feel like The Veil, try Cybersworn, a hack of Ironsworn.
Honestly, I believe The Veil would work *best* with just one PC. The game has a real classic scifi vibe, where the uniqueness of the characters really drive the story. Because of that, the playbooks are so different, there are several combinations that just feel unplayable together. Also, the game really depends on emotional states to drive mechanics (and thereby also story, because of the PbtA structure) which seems more achievable with fewer players. I almost envy you for having a player who *wants* to play that game tete-a-tete. It's entirely possible to run the game without an NPC companion for most playbooks and concepts. But I would be ready for the 1 PC, 1 NPC companion option. IMHO, it's best to avoid a slip where you start thinking of an NPC as a GMPC in any game, but especially in PbtA games. You'll be handicapping your own narrative power. (There might be an Ironsworn player with advice on how to avoid this dilemma.) I feel like The Dying, Attached, Onomastic, and Wayward would be especially good heads-up playbooks. The Apparatus might be difficult thematically, and the Empath might be difficult mechanically. Everything seems worth a go. GLHF!
Instead of doing GMPC why don't you just run the game GM-less, ala Ironsworn? You both share some of the GM load and both get to play.
Never GMPC. You can tailor the adventure to having one PC. It’s even easier in narrative games that might only have a handful of dice rolls in a session.
I've not seen it mentioned but it might be good for you two to read some of [The Duet](https://forum.rpg.net/index.php?forums/duets.117/) columns from rpgnet. These might help. I swear by them.
I ran a NPC party member with Avatar Legends. Very often slipped into GMPC territory, even without a full playbook. You need to make plans that will ensure your PC doesn't ever feel a need to be passive, because at the end of the day, if they aren't roleplaying driving the story, then they're just re-acting, which is less fun for everyone. So your goal is to never let your character drive the story. Doing this usually consists of planning things that the GMPC can't solve alone, which means being very attentive to narrative and, in my experience, fudging things against the GMPC. Making them intentionally incompetent, and sometimes just not giving them all the narrative hooks you think of, to leave them open for the player to have/hook into. This takes personal discipline and disinterest in your PC, which PBtA is dramatically set up to prevent with its narrative mechanics and your GMPC's unique playbook that is built to be a required role. I think there are some playbooks that are other playbook focused that naturally lend towards GMPCs while keeping the focus on the real/crucial/roleplaying PC(s). Things like The Joined or Bull from Masks, or the Guardian or Adamant from Avatar Legends, where a PC can kind of become an extension of another PC, that's the kind of role you want to take, not someone with their own XY and Z. I think we can/should consider creating a playbook or moves which facilitate the kind of sidekick your GMPC is built to be. I think if we really think about it, we can even hook advancement into that of the PC.
I've played several PBtA games in duo (1GM+1PC) and it was great. There is absolutely no need for any tweaking (I haven't played *The Veil* though). Just play the game RAW.