Companions. Doctor Who is famous for them - Leela, Peri, Sarah Jane Smith, Adric, etc, etc - but almost everyone has them. In some cases they're sidekicks of a sort for a single main character. In other cases a larger group sticks together. Frodo has Sam. Aang has Katara, Sokka, Toff (and Appa!). Zuko has Iroh. The list could go on and on.
Why are companions so important?
One reason is social realism. There aren't that many complete loners out there. People have friends that they live their lives with.
Another reason is that the main character needs help. When you look at the Avatar group (Sokka wanted to call them "team Avatar" I believe), it's balanced between different types of people. There's an air-bender, a water-bender, an earth-bender and a warrior. That gives them a wide range of skills and strengths that they can use to get through their stories successfully.
Another big reason is information management. The Doctor has mountains of specialized skills and knowledge - because he's a Time Lord! - but without the companions he'd have no reason to explain any of it. If you have a major character who's an incredible specialist on some topic, you can always show him or her doing what he/she is good at... but if you build in an information imbalance between that person and someone else, it gives him/her an opportunity to explain where that skill came from, or how it works, or any number of other things that would otherwise feel like blatant infodumping.
Conflict is another reason. Conflict can serve the purposes of information management, as when two people start arguing and that lets them divulge information to the reader that the characters already know (without using as-you-know-Bobs), but I've separated it out because it actually does a lot more than that. Conflict is an enormous source of drive in the plot. Ongoing disputes (of the right variety) between a character and her companion can influence where the story goes and keep us wanting to see what happens. Conflict can also drive character development.
Dealing with an introverted character is a lot easier if that person has a companion. You can make good use of internalized thoughts when you're working with the written rather than the visual medium, but still, internalization can only take you so far. A companion gives the introverted character a reason to try to speak - or perhaps a reason to try not to speak! A companion will bring certain topics into the introverted person's thoughts. Appa gives Aang a reason to talk out loud even when he's alone, which is very useful to the storyteller who can't make any use of internalization.
Companions also create wonderful opportunities to explore language. Some companions maintain an ongoing banter which can really add to the ambiance of the whole story. Their talk can be helpful for a story not only for content reasons, but for dialect reasons, and for the way it reveals aspects of the social contract in the community from which they (or each one) comes.
I'm not going to end this by saying you need to go off and give your protagonist a companion. Sometimes that's the right thing for a story, and sometimes it isn't - but it's worth considering. Even if the companionship is short-lived within the story, it can still be a valuable addition to what you're creating.
Chances are that if you've gotten much of a story written (especially a novel) you already have companions built into it. If you do, then it's worth looking at them and thinking explicitly about how they are functioning and what kind of work they are doing for you, the writer, as well as what they're doing for the other characters. That way you can deepen them, tune them, and strengthen them so that they're making a bigger difference for your story.
It's something to think about.