Okay, so in my story, a character goes missing, however you never actually meet them. How can I get readers to like them even though they’re never even introduced to them?
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This literary trope is called "Posthumous Character," and although your character isn't actually dead (presumably), the same idea applies. You are trying to get your audience to like, understand and see a character who is not actually present in your narrative.
Therefore, this absent character will probably be developed entirely through flashbacks. This is hard to do properly, of course, and it has a big drawback - it has already happened, after all, so it can't really affect anything in your present-day plot. But if you're trying to develop a missing character, this is really the only tool for the job. Other, related options could be parallel story structure, where you tell both the current-day story and the past story of this missing character's adventures.
Movies do this "developing dead/absent characters through flashbacks" kind of thing all the time. When a character has a tragic backstory, i.e. a missing wife or deceased children, the movie will flash back to a time when those characters were alive, showing us how kind and loving the wife was or how wonderful the children were. You can mirror this kind of nostalgic flashing-back in your story, if you want to develop absent characters without actually having them be present.
However, are you sure doing this is a good idea?
Why you shouldn't always develop absent characters
Developing characters who a) aren't actually important to your narrative, b) will never show up, or c) are dead, is often, to put it bluntly, a waste of your time. Why spend your valuable word count developing a character who we will never get to meet? Why should your reader care about somebody who never appears except for flashbacks, doesn't help the protagonist in any way, and basically never affects the plot? I'm guessing that you could cut this character out of the story entirely and not affect anything at all, and that's never what you want.
(For reference, consider the Sexy Lamp Test. Could you replace your character with a lamp and have literally nothing change?)
To make the exploration of absent characters count, then, tie it to a character who is present. A dead wife isn't important to the story unless we learn how much her living husband misses her. A missing son isn't important to the story unless we know he makes our motherly protagonist cry at night. And so on.
In short: You can absolutely develop absent characters, using flashbacks and other tropes. However, be warned: without having a solid, strong narrative reason for why developing this absent character is important, all you will do by developing them is waste your time and word count. Your effort is probably better spent on your actual characters and protagonists, not one that the reader will never meet.
Show how they matter to people:
I have a major character in my novel who is dead, and appears only as a spiritual entity. The ghost interacts with people by sending dreams and visions. She is also one of the most sympathetic characters in the story. Sendings and visions are not a material way of showing up, but can work. Dream sendings operate like a flashback, introducing the character with things that happened long ago.
Sometimes you have someone who will show up later in the story and play a prominent part. You want your readers to know them and to know their importance. Relate the character by the actions of the other characters. Have them talk about the missing character, remember the things they did together. Have the things they do center around helping the missing character, or have them say how the missing character changed their life.
Try to imagine The Lord of the Rings with Gandalf as an unseen presence. The characters talk about him, refer to the things he is doing and has done, how he changed their lives and fought evils. Without actually seeing him, you can see how you can set him up as a great hero and savior. If/when he shows up, you wouldn't need to develop his personality unless you wanted it to clash with the vision you had created.
The absent character makes itself felt through their material or spiritual connection to the protagonists. For example, a missing senior hunter or warrior would always be in the minds of their junior friends because of all the techniques and valuable survival skills they taught them. When stuck, they would brainstorm "what would X do here?" which enables them to shed light from a different perspective and helps them to prevail.
If the absent person was good at crafting the protagonists would wear clothes and gear that helps them on a daily basis in their adventures.
There may also be an exclusively spiritual bond: A love or deep friendship, an oath of loyalty, something they achieved together — any and all of these can be a source of mental strength which keeps them going in the face of adversary and makes clear why the missing character is important to them and why they revere them.
If your POV character has a lot of affection for the missing character, that will tend to shape the way your reader feels about that character. But it needs to be organic, and not too heavy handed. Otherwise it will backfire. Showing the ways the missing character leaves a hole in the POV character's life is likely your best bet.
There's a great example in the movie Stand By Me. The POV character's older brother is deceased. He only appears in one brief flashback, but that demonstrates clearly how he shows the love for the POV character that the rest of the family doesn't. He's also referred to frequently by other characters in ways that highlight how much they liked and respected him.