How do I incorporate powerful allied NPC heroes in a fighting scene?

Question

I understand what an powerful allied hero should do in a fight to make the fiction interesting (be powerful but not quite powerful enough to get it handled all by themself; be powerful but ignore parts of the situation etc...). What I have trouble with is: How do I incorporate these things in the flow of the game?

Let's get a bit more specific with an example:

The Legacy's dad is a powerful and well respected super hero. The big bad villain is the Legacy's family arch-nemesis and is about to attack their stately home. The Legacy, their team and their dad are present to defend it.
I start to describe the scene. What the Villain does, who their allies are and what they do and how the Legacy's dad is fighting them. So far so good. The dad can be presented as a strong hero here who is simply outnumbered due to the huge amount of allies of the Villain. So the PCs have to step in and help.
But during the play I have trouble to keep this narrative alive. I start with "What do you do?" And the players describe their actions. They trigger some moves (Assess, Unleash, Engage...) and we play them out. They either succeed or they fail. In the latter I make a GM move and let something bad happen. Then I go back to asking "What do you do?".

My issue now is, I have limited agenda over the fiction when the players succeed with their rolls. The only times I'm able to come up with stuff on my own it has to be something bad. Sure, I can make the Villain badly beat up the Legacy's dad and that feels like a good GM move. But if that's the only interaction the Legacy's dad has during the whole encounter, it makes him feel weak and useless.

tl;dr: How do I make the NPC hero feel strong while following the usual flow of "PCs trigger moves that succeed and they do something cool, or they fail and the GM does something horrible"?

Answer 1

Make adults seem childish and short-sighted.

Right, so stately Huma Manor's under siege by Dr. Scaravich, his battlesuit, and his army of robot scorpions, and Dad Huma is teaming up with Son Huma and the rest of the PCs to beat them back, and you're worrying about what actual role Dad Huma is going to play in all of this.

So, going with what you've said about the situation, start with what would happen if the PCs weren't there: Dad Huma would fight Dr. Scaravich to a standstill, and the robot scorpions would only have to get lucky once.

Then, filter through that principle in the title to get an idea of how Dad Huma is going to react to the PCs. Dad Huma's not there to uncritically back up Son Huma, he's there to do the things Dad Huma wants to do, because nobody can tell him otherwise. Is he going to treat the bunch of outcasts and bad influences Son Huma is running around with the same way he treated the bunch of outcasts and bad influences he ran around with at that age? Learn their powers, understand their weaknesses, try to make teamwork happen?

Of course not. Things are different now, because reasons. Just try and keep those robot scorpions off him and stay out of his way. Surely you can at least do that.

So now you're at an interesting spot where Dad Huma and Son Huma are fighting for the same things but not in the same ways, and that's liable to cause all kinds of problems.

You don't particularly need to endear Dad Huma to the PCs, just play him honestly. Son Huma is stuck with Dad Huma no matter what happens, and to everyone else Dad Huma is just another adult whose desires conflict with what they want for themselves.

That said, what do you play Dad Huma as doing?

Dad Huma, The Power

One of the tools in your GM toolbox is that you're the central arbiter of the fiction. Moves need to be happening in the fiction in order for the dice to come out and augur the results, and the results must follow from the fiction, so a move like directly engage a threat gives you throttles coming and going.

Could a PC just step up to Dr. Scaravich's battlesuit and engage it? Maybe not - that's a common thing for large singular threats, making them need some work on the PCs part to create a vulnerability before anyone can fight them directly. Maybe so, but when you trade damage you manage to scratch the paint before you get punted into the air and 20-comboed by the point defense lasers.

But when Dad Huma's also fighting it, that changes things. You don't have its full focus so it will do less to you, and you don't need to deal meaningful damage as much as you need to distract it so Dad Huma can win an exchange.

Of course, that might not really establish Dad Huma unless somebody tries to step to Dr. Scaravich when he's not around? But it can still happen.

Dad Huma, The Threat

This isn't to say that suddenly Dad Huma's going to start hunting down Son Huma's team, but more that, if Dad Huma is that strong, and not particularly concerned about how all these interlopers in the fight are doing, the side-effects of his fight can be something you draw from to threaten the PCs. Dad Huma pitches a robot scorpion in a random direction. Dad Huma rams into Dr. Scaravich and the battlesuit goes stumbling back. Dad Huma throws around bursts of blinding light.

Even if Dad Huma's not the sort of person to be deliberately careless, the fight is easily intense enough that he can't be perfectly careful, either.

Dad Huma, The Threatened

I understand not wanting Dad Huma to just get shot by Dr. Scaravich's Petro-Ray. But the reason it probably feels off with you is that it doesn't feel like a fitting way for Dad Huma to get hurt. Dad Huma should get hurt in keeping with his character, and I don't know what that's going to be for you, but one of the primary Legacy strengths is Savior. So, following what a high Savior makes you good at:

Dad Huma doesn't just get shot. Dad Huma gets shot saving you. Fixing something in the manor you accidentally broke, punching through the pile of robo-scorpions that swarmed you, taking the blast from Dr. Scaravich when you finally got his attention and couldn't get out of the way.

The result is the same (you rolled badly, Dad Huma takes it on the chin) but the fiction fits Dad Huma better.

Answer 2

Only narratively, since the game has no mechanics for that

In most cases you just leave that off-screen. A powerful NPC fighting along with the party is (arguably) considered an anti-pattern, since players often are not happy with such kind of NPCs. D&D-like games struggle more because of it, while PbtA mechanics deal with it quite well because of player-driven moves.

There are two playbooks in Masks though which at first glimpse assumes such a character:

• The Protégé has their Mentor — a capable and experienced hero
• The Legacy has their, well, legacy — heroic ancestors of the past

But none of these books asks GM to actually incorporate such a character into a fighting scene along with PCs. The Legacy's ancestors might be far away or even dead, while The Protégé's Mentor acts as a teacher and, well, mentor, rather than an active combatant.

The reason is simple — the game has to be a story about PCs themselves, not about a PC's dad fighting his arch-nemesis. "Make the player characters’ lives superheroic" and "Play to find out what changes" agendas are mostly about that.

All tangential rather important events GM is supposed to leave off-screen. See the "Think in the gutters between panels" principle:

If it makes sense that the Protégé’s mentor would’ve made their own attempt to take down the Magus, then make your moves and say what happens.