My Character's Path

So I need some help coming up with a plot… for my main character

My story is intended to be in two parts with three main characters. Part one focuses on when they first meet at the age of 12/13 and, while not fully fleshed out, I have an idea of how it all goes. For the most part, the three stick together throughout so I don't have to come up with entirely separate plots for them.

The second part, however, takes place ten years later. The deuteragonist and tritagonist become warriors, so there's plenty to write for them, and I have ideas on what to do. But the protagonist's job requires him to stay at home in one room for the most part. He's an inventor and weapon's specialist, and he stuck making weapons basically all the time. My thought is to exclude him from having a plot and just focus on the other two. I've always intended on bringing him in the final act in an important role, so maybe it'd be okay to have him be absent before that? Aside from his interactions with other main characters. My issue with this is that they all grow a lot in the time skip, and his role in the final act requires knowledge of how he's changed and why he's so important. But how can I show this when he's essentially always stuck in one place?

Another idea I had is to maybe have him come out for certain missions, but it'd seem out of character for him. Or maybe he'd be like the NPC who needs to be escorted, and those escorts just happen to be the main characters? I'm just really not sure. I've toyed with the idea of just dropping the inventor thing too, but it's just such a huge part of the story that dropping it would essentially mean scrapping the book. Any advice?

You could follow your protagonist's path to becoming a weapons specialist instead of just having him already there doing his job. If you think it may confuse your readers to have a different timeline for him, you could use flashbacks or write him a separate novella (although I can see why you wouldn't want to do that). Also, is his staying in one place vital to the plot? If not, he could travel around for his work which would be more interesting to write/read.

@Wry_Wyvern It's not entirely vital, but it makes the most sense? He's special because he discovered a way to make weapons harnessing a certain kind of magic people hadn't been able to tap into before. The issue is that he discovered it based off his father's (who was also an inventor) notes. In the beginning when he first came to show people what he'd created, other, more experienced scientists/inventors wanted to take his notes and develop the weapons themselves (with the ulterior motive of calling it their own), so he's kept them to himself and won't let anyone look at them, which has the downside of him having to restrict how many people can work with him.

He COULD probably travel around, as he makes his weapons mostly out of recycled materials and would have to go places to get them, but I feel like those kinds of plots would be kind of unnecessary. But now that I think of it, I realize I can probably have him involved in some sort of political plot. Like, because his role as a weapon maker is so important, he would probably have an important role in the community he makes weapons for, especially if he's created this sort of monopoly on this particular weapon. And maybe I could have a whole conspiracy thing going on? I'm not sure, but I can work out the kinks

That sounds interesting! Good luck with your story!