Plot-centric NPC help for a homebrew (bard plus some unique abilities)

I'm creating a plot-centric NPC bard (subclass undecided) for a homebrew I am working on.  This character was made from a dragon's soul (he doesn't know this) and may unlock dragon abilities at some point in the campaign.  Any advice on how to manage this while keeping things balanced in terms of character stats/abilities?  I've never made a major NPC before, I've only DM'd one shots from pre-written modules.  (I'm making him a bard because it ties well into the NPC's story/circumstances.)

I thought about just giving him one or two big fire spells (or whatever is relevant to the dragon type I end up choosing), but making them once-per-LR or something along those lines, but I wanted to hear from experienced DM's, since there might be a better way to do this.

This NPC is locked into the core of the story, so whatever happens to him will have major world consequences.

If party makeup might influence your suggestions, I will be DMing for 3 players.  A rogue, a cleric, and the third is undecided.  The third has moderate experience, but the rogue and cleric are newbies (I've DM'd a couple of one shots for them, but game mechanics are still super fuzzy, so they need a lot of guidance).

Feel free to offer general advice as well, as this is my first full campaign.  I am trying to keep the plot from being overly complicated, and the setting is localized to a city and *maybe* the local wilderness, depending on player choices.  But I'm leaving most of the world open ended in case we decide to play more after our first adventure.

First advice is, don’t think of him in PC terms. He’s not a PC so there’s no need to limit yourself that way; NPCs don’t have to follow PC rules. Just give him whatever powers you want him to have. Even a step further, if the characters aren’t going to fight him, you don’t even need stats for him at all. But if they are, stat him like a monster, not like a PC.

The second advice is, be very careful about making one creature plot-centric. I’m not sure exactly what you mean by that term, but it can be risky. Just realize that in D&D, a lot could happen: he could die, or get forcibly sent to another plane, or get polymorphed into a goat, or 100 other things. So make sure the story is durable enough to withstand losing him in one way or another. Because if you just give him plot armor, the players will notice, and the game will suffer.

Also, I’ve got to say your last paragraph is perfect. That’s the advice I’d give every starting DM.

Thanks for the advice, I appreciate it!  I like the idea of using monster stats, and I didn't really consider that he didn't need to follow PC rules, TBH.  As far as plot centric, I guess that's not worded that great.  He ties into the plot, but there are a ton of directions it could go, including him becoming an ally or an antagonist, and the greater plot would definitely still function without him.  Part of me almost wishes they would fight him >=) , but knowing my players, they will want to befriend and help him, which is also good.  But yeah, the game could definitely go in any direction, since the players are still the ones who have to solve the problem, not the NPC.  I designed the NPC for the story, but he's not the key to completing anything. 

I have a few different possibilities in mind, but I'm trying to design it in a way where I could change things up on the fly if necessary.  The first time I DM'd was from a one-shot module, and the players ended up saving an NPC that was written to be a murder victim, and also intercepting the BBEG's helper, who was written to be part of the final boss fight, so it was really interesting the way things turned out, and really kept me on my toes because they did multiple things that were not accounted for in the written material.  It definitely showed me that you can try to predict player decisions, but you have to be ready to make big changes and improvise at the last minute.  Kinda of thrilling and terrifying at the same time, lol.