I like the Handlers, and I like the label as a framing device, and it makes me think about what other framing devices I can use for NPCs who are more than NPCs but less than GMPCs. Part of it is about making them easier to create or use or make interesting, part of it is to make it more obvious to the party what their value is, so that they are actually useful, but don't take away agency from the party.
I've decided to call this category of NPCs Specialists, the idea being that they have some highly specialized skillset or knowledge, or are in a unique situation, which would make them useful beyond what any party would likely be able to do on their own (at least from the start), but are otherwise not well-rounded enough to be adventurers themselves (or are, due to their circumstances, otherwise indisposed). You might think in some cases that they're taking away from challenges the party could face for themselves, but I would argue it's just changing the nature of the challenge. I don't like the minutia of mapping a dungeon, but finding the best Cartographer for this dungeon and keeping them satisfied is much more appealing. Cartographer A is more experienced, but hates spiders. Cartographer C is the best, but they won't accept any job unless they can 100% map the dungeon. Cartographer E is as good as the others and cheaper, but he's really, really gross and annoying. These are more suggestions than hard-rules, have fun with it!
This post was loosely inspired by a comedy special that just came out on Netflix from Bob Rubin. It was a bonus episode attached to Patton Oswalt's new special. Bob is... interesting. I enjoyed it, but I don't know if I'd recommend it to everybody. I don't know if I'd call it "good" comedy, but there were moments of brilliance. Anyway, he has a whole bit about how when you own a house and need housework done, you get a contractor, but when the contractor needs to get something specific done, they get a sub-contractor. And these sub-contractors are all crazy unreal cartoon people who can barely function in the world, but are singularly masters at what they do. It was one of the funnier bits, and you could take his sub-contractors wholesale as Specialists.
The Handler is wealthier, better connected, or more experienced than the party. They are employers or managers, or agents or consultants. They aren't necessarily adventurers themselves; maybe they used to be but retired from the job, or worked their way up the hierarchy, or got injured on the job. They might come along in a pinch and are quite skilled, although a bit rusty or past their prime. Or maybe they were never adventurers but have a knack for management. Or maybe they're bad at their job and got it through luck or nepotism but the party is stuck with them.
Wizards and alchemists are smart, but adventurers necessarily require a breadth of knowledge. A Scholar may spend as much of their life studying the mechanics of a single spell as an adventurer wizard spends across all their studies in their entire career. If circumstances require hyper-specific knowledge of a common topic, or knowledge at all of an obscure topic, you'll need a Scholar. However, one does not become a Scholar unscathed. They are often eccentric and asocial. Or they're so obsessive about this one thing that they're boring and intolerable to deal with. Or, despite their intelligence, are shockingly ignorant of basic things. Or they're actually really charming, likable people who happen to be brilliant and interested in this specific thing.
You want to build a castle? You want to fortify a settlement? You just want to be comfortable? You call The Builder. Probably you call a specific Builder for a specific task; the architect is not the carpenter is not the interior decorator. The Builder may build secrets into your project that won't be discovered for a hundred years; they might do weird sex things in your soon-to-be-home when you're not keeping an eye on them. But they're the only ones who can get the job done.
Did you know that adventurers used to draw their own maps? Can you imagine some dumb oaf fighter, or aloof day-dreaming wizard, trying to draw their own map? And the rogue is just gonna make it up and "commit it to memory" so they can take all the treasure for themselves (or more likely get blackout drunk and forget anyway). No wonder they kept getting themselves killed! No, if you want to get the job done right, you bring the Cartographer. Sure, you've gotta keep them alive, and keep them moving when they want to draw every little detail (although you might want to give those details a look now and then...), but if you want to get out alive (or ever come back), do yourself a favor and bring a Cartographer.
If you're bringing a lot of stuff, or expecting to take a lot of stuff out, get the Movers. Sometimes it's just one big person, but usually they come in pairs. Some stuff is just too unwieldy for one big to move themselves. Sometimes the Movers are scrawny teens way out of their element who just desperately need the money. Those ones don't last long on adventures. The thing with Movers, is they tend to be the sorts who have absolutely no time for your shit and will be completely unhelpful beyond the scope of their job even if all they have to do is swat off a measly goblin, or worse, they think because they're big and strong that they're adventurers too. They're not, but still, it's impressive just how much they can haul.
(Wolf Pack and) The Cub
This child, alien, small mythical creature, other naive and relatively unskilled person is important. They are the key to the magic McGuffin, or an important royal, or a witness to a crime, or the argument for or against some larger cause. They must survive, at all costs, even as they seem desperate to get themselves killed. They may be helpless but loving, or capable but an arrogant, immature pain in the ass. But you're stuck with them.
The Glass Cannon
They are incredibly powerful. They are the only memorizer of the Magic Mega-Missile. The only operator of the hyperbeam quantum pluscannon. The only one who can pull Excalibur from the stone or lift Mjolnir. But they only get one shot, and they aren't much good for anything else. Some Glass Cannons are absolute cowards and need to be coaxed along every step of the way. Others don't know they're Glass Cannons. I don't know which is worse!
Not all problems are solved by swinging swords and casting spells. Sometimes a little more delicacy is required, and adventurers aren't exactly known for their delicacy. So when boring politicking is on the line, just chauffer the Negotiator, keep them happy and keep them alive, and let them do their job while you have fun at the Fancy Tavern. But keep in mind, for as charismatic as they are, or genuine and lofty in their ideals, they are prima donnas. For them to do their job, they need you to follow the contract to the letter. No brown-colored sugar-coated chocolate candies in their candy bag, or the deal is off! Or so they say. But they look pretty genuinely distraught about it...