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 them self; 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 heroes have to step in an 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 (Asses, 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"?