I have long been of the opinion that if I get through the first quarter of a book and I can't think of a single character that I care about I should stop reading. (Although, in practice I usually finish any book I start) One of the things that I have found is that when I'm reading good authors I find myself having imaginary conversations with them. Telling them about the one piece of information they are missing, or how that other character really feels, etc. My favorite example of this is Lois McMaster Bujold's Vorkosigan's saga. I have been through those books over & over again. I still find myself talking to Miles (the MC) and will still tear up over certain passages. So what do YOU do to make people actually care about your characters?
I try to trigger an empathetic response in the reader by having my characters experience familiar situations - from war and injustice to small things like waiting in line at the DMV. Humans also have the tendency to want to piece together information and fill in blanks. I utilize this by slowly revealing specific information over time in a way that allows the reader to read between the lines. This works extremely well for characters that need to eventually be killed off.
I never thought about this but I believe this is what I do. I try to create characters that interest me and that I can become emotionally invested in. If I have my MC get slammed into a wall by the villain or punched in the face I want to know him/her well enough (and like them enough) that I feel guilty for having made it happen. When they are feeling complete despair and sobbing with defeat I want to feel a desire to comfort them. If I can do that...bingo.
Giving them likable qualities, I suppose. There're probably no universally-liked qualities but making them funny/clever, or caring about their friends, hardworking/honest - kinds of things you'd like in a person you met, really. You don't want to overdo it and make them TOO 'likable' because then they're just unbelievable - "okay, no one is that nice". Obviously there have to be negative and neutral qualities too. I have a friend who's a grade-A asshole in some regards but really generous and sweet in others and I always wonder if I'd like him if he were a fictional character, hahah. It's a balance.
I like to take something familiar and something impossibly horrible and smush them together; two (or more) opposed stressful things happening in concert and keep going until they start to come apart. I'm always looking to find conflicts and contradictions and making them butt into each other. Because it turns out out these horrible 'how could you!' situations are always strangely familiar; playing off lies and shame and things we understand but like to think we're above. My current project is about a teenage girl in the typical enough life. Her parents split up acrimoniously and that really effects her life, suddenly coming home to an empty house. She finds a boyfriend and all that comes with that for a teenager. Oh and she somehow ended up telling everyone at school that she has cancer. And she has to keep that (somehow) from her mum and her boyfriend. She's lying and pretending to everyone in her life, can't even tell them why she's crying and she's slowly loosing who she is in all the deception. We care about her because despite how different her problems they stem from problems we know; we've all had a lie get out of control. We recognise how much this hurts and know everything she's struggling against and how it's all stacking up on her and how much she wants to come clean. Just having that one, huge, impossible lie come crashing through the wall means we see the real her, someone that is damaged but not unsavable and that's something that everyone deep down understands. We care about Beth, despite her lies, because her pain reflects the pain in all of us, even if we can't put ourselves in her shoes. Which is a long way around to say I really believe in going further, way past the point of comfort, cracking under the weight of problems they caused for themselves. Only by going beyond the day to day, past the skin deep can you find something that speaks to the universal parts of humanity. If a character does that then we'll care about them no matter how terrible they are or how distant from us their situation. We care for them because they are us.
I don't want to confuse likable with caring. When you read books like Lord of the Rings you care what Gollum does & you might care what happens to him but you don't like him. Same can be said with any good evil MC in a horror story. My problem is apathy, I don't want to write characters that are unrealistically good or unrealistically evil. My characters are supposed to be normal people that sometimes do some shitty things to other people but only because that's what a lot of people do. They look out for themselves & their families at the detriment to others. The issue I'm worried about is that can make them dull as characters.
Why? Just watch some documentaries (I can recommend some, if you want drop me a PM) and you will discover that these kind of characters abound in the real world too. You just have to get into their heads and let the reader take it from there. Trust me, the reader will get it if you know what you are writing about..
Yeah - I usually create people I want to hang out with and then put them in interesting, morally-conflicted situations that raise a lot of questions (for my mains, this usually means family tension in the backstory). Either that or I create people that have gone pretty far off the rails and messed up their lives before the story even starts, and you wonder how they got that way (that's a few side characters and my villain).
I would propose a combination of what other people have said. Likeable qualities, relatable situations, care in writing, and insight into their thoughts and feelings can all make characters more sympathetic and important to the reader.
I try not to cross a line into making my mc a total bastard. The anti-hero can really be overdone. In my WIP my characters are prisoners in jail. They're out to control things. They use demeaning techniques, threats and violence. They're not nice people. But they want things ... things people can relate to - personal dignity, privacy, kindness. It makes them vulnerable. Very helpful for a tricky characters. I don't know how my mc is likable but I root for him. He wants to love something.
Loving something is a very powerful, relatable and human motivation. He sounds vaguely like my character Dale, who has something like an Absent Parent Syndrome, where he is usually eager for and attached to loving bonds. I love Dale. He was the first character of mine to make me cry.
Three-dimensional, fully fleshed characters are all you need. People's tastes in what they like in a fictional character, and how they feel, alter across the world. As long as the character feels fleshed out, has motivations, etc. They'll be liked by someone. But, as pointed out above, you should write interesting characters. If I feel like the author is trying to manipulate me into liking their character by going 'look how hard they've had to fight against the System, they just want to Help the Helpless' I tend to roll my eyes and feel myself pulling away from the character. Give your character hopes and dreams, motivation, wants, desires. Make them feel real, and people will like them, or not like them, but at least they'll find them interesting. There are many characters that I don't particularly like, but that I find interesting. One thing I will say, though, is much of what I like or don't like about a character is how other character react around them. I strongly dislike Benedict's Cumberbatch's Sherlock Holmes. I loathe him. I don't mind the actor, he seems like a decent guy, but I loathe his Sherlock because whenever I watch it, all the other characters let him walk all over him. 'Oh, but he's a genius so he can treat you like an asshole'. It's two-dimensional writing, but I will give that Moffat is good at dialogue. He'd be far more interesting if people dug their heels and went, 'no'. And I mean, a good hard, strong no. Not a 'no' which later turns into an 'oh, well, that's just him. He's just an asshole' with no consequences. Because let me tell you, if someone went around treating the police like that in real life, one of two things would happen, they would lose their position, no matter how good they may be, or they would be arrested for charges that were generally overlooked previously. Plus, Lestrad by reflection, because he allows Sherlock to being an asshole and walk all over him, looks like a bumbling fool by comparison. Isn't he meant to be a decently high up detective? Not one who can't solve a single thing without Sherlock and sits there allowing abusive comments thrown at him. That's just my opinion, however. Though, Sherlock has flocking fangirls by the millions, so maybe I'm wrong.
I think the thing is that they're so used to it that they just tolerate and seethe under the surface. And John gives plenty of the audience's reaction representation in it.
A believable character who wants something badly (embodied in at least one believable character). There should be something stopping them (embodied in at least one believable character).
I find that readers care about characters who themselves have someone or something they care deeply about. A character who wants to become wealthy because they're a lazy hedonist might be cool, clever, or funny, but readers probably won't be too invested in their ultimate success or failure. (Indeed, part of the fun of that character can be watching how they mess up their own plans and end up back where they started.) A character who wants to be wealthy because they nearly starved to death once and never want to be in that position again may be much easier to root for. Same for someone who needs money for their father's medical expenses, or to pay for music school, or to provide a better life for their child.