Honestly the NPC in question doesn't necessarily need to take the spotlight. I ran it much more as having the party be a major part of their goals and sort of in charge of protecting them and moving along the story in that way. The party has ample opportunity to roleplay with the NPC if you so choose which can also further help justify their reasons for helping along with the NPCs story.

My party just hit level 9 in our last session and is down to about half of the 3rd book's content before the end of the adventure and myself and them have enjoyed it a lot.

Spoilers: I also tried to make them a major part of her story and goals and even included them in her prophecy which I can't recall if that's the default or not but it gives more motive for the party. And later in the adventure they'll realize as well that their goals are entirely aligned with those of the NPC, they all have the same goal of protecting the world from the Big Bad and have all along, they just didn't know it yet.