Most of what I write essentially involves one or two key characters who, through choice or circumstance, have to go out and do hard, heroic things. Of course, sometimes these hard, heroic things aren't exactly heroic. They might be self-serving or even downright criminal. But the key thing is the characters themselves believe in the fundamental rightness of what they are doing, no matter what else the world tells them.
However, in most cases, they can't do it alone. Even the most die-hard of "loner" characters have a deep bench of help and support characters to get the job done. Batman can't Batman if he doesn't also have Alfred, Jim Gordon, Lucius Fox, Dr. Leslie Thompkins, Barbara Gordon and Dick Grayson in the mix with him.
I've been thinking about the archetypes of these support and infrastructure characters, how I've usually expressed them in these works. It's not been a conscious thing, a checklist of, "Oh, I need this"-- but when I look back at each works' set of dramatis personae*, I see most of the archetypes represented. Of course, each of these things aren't limited to one character each: one character could fill many roles, and some roles can be fulfilled by multiple characters. This is hardly a fully representative list of "this is what you must have".
A Hero needs someone who knows the things they don't. Heroes that know everything are not particularly interesting. It's good to have someone in their corner who gives them information they specifically need to figure out what do to. Not necessarily someone to put the pieces together for them, but someone who can say, "Here are the pieces for you."
A Hero needs someone to tell them hard truths. Any hero that's worth reading about is going to stray from their path, get their moral compass out of alignment, or otherwise have a blind spot. Someone needs to be there to smack them across the head and tell them the thing they won't tell themselves. Things that they don't want to hear, but probably need to.
A Hero needs someone with authority. Much like the previous point is about keeping the hero in check, I think a hero needs to have to answer to someone, in one way or another, to keep them from just having carte blanche to do whatever. An unrestrained hero is also uninteresting.
A Hero needs someone to patch them up. This is twofold: someone to physically heal them when they get hurt, and someone to pull them out of the abyss when things go wrong. The first is just common sense, and doesn't even necessarily involve a character so much as infrastructure. But a specific character is probably more interesting.
Do not use this as a checklist: that would probably get you in a lot more trouble. I may use the word "need" a lot in this, but take that with a grain of salt. Above all, a story needs what it needs.
There is one deliberate omission, something I was going to put, and then erased, but it's worth mentioning: Someone with loyalty. I mostly erased it because, to a degree, that loyalty is implied in all the other elements. But I also erased it because it implies that the loyalty to the hero is something that is deserved rather than earned. The hero must earn that loyalty. The hero must KEEP earning it.
Plus, the conflict that arises from a rock-like loyalty being shaken? That's real juicy stuff to write.
*- I always write up a Dramatis Personae for each book, mostly because I need a quick reference for the names of minor characters.