1) Show me a distinct personality
Show me an aspect of their personality that makes them distinct from the 'stock human'. To me, every character is the 'stock human' when I first encounter them (even if they're not human). Background characters are 'stock humans'; the cashier that checks out the protagonist's shopping, the guard that tells the protagonist to state their business, the innocent bystander that runs away in fear when the villain attacks the town with a doomsday weapon.
Basically, a 'stock human' acts how you'd expect a person to act to any given situation if you had to guess without knowing anything about their background.
I start caring about a character when they break out of the mold of a 'stock human'. When the cashier asks why tf you're buying 100 packets of gum, when the guard tells you about that time they took an arrow to the knee, when the innocent bystander is in awe at the doomsday weapon and wants to get a closer look. When they do something that makes them feel like their own person.
This often means I tend to connect quickly to 'tropey' characters with an extreme personality, but that doesn't mean I can't connect to more lowkey characters either. But if your protagonist has a more subtle, 'normal'-seeming personality, ffs don't introduce me to them getting bullied and being sad about it, or grieving over the death of a loved one, or basically being in any situation where they'd act like you'd expect a normal person to act :'D Introduce them making a choice most normal people wouldn't make, or being put in a situation that's uniquely emotional for them, but mundane for everyone else.
I also feel like this is why often a lot of people care more about side characters and villains than the main character, because they tend not to be the 'stock human', whereas the protagonist is often the 'everyman'.
2) ... but try to make it believable
An extreme personality would grab my attention, but to keep my attention, you'd have to convince me that this is an actual human being (psychologically, if not physically) and not just a puppet that you can make behave however you want on a whim.
Sometimes, if they don't have that much screentime, all you need to do is make them behave consistently; show me a pattern to their behaviour that I can extrapolate and speculate about, even if you don't spell out their internal motivations and everything yourself. For instance, if your side character has anger issues, and also loves cake, and also has a fear of clowns, and also speaks 5 languages, it'll start feeling like you're just making this character do whatever you need someone to do for plot reasons. Keep it to one or two core traits, if you don't plan on exploring this character further
On the other hand, if your character has a lot of screentime, having a core trait without exploring the underlying psychology will get old and gimmicky very quickly, and that's where you'll have to show me more under the hood for me to keep caring about this character instead of getting sick and bored XD A minor character can get away with an unexplained gimmick because you could plausibly imagine that they'll do stuff offscreen that connect to and explain their gimmick, but if you're following a character for most of their life and just see a trait in isolation pop up over and over again, they're gonna feel like a 'stock human' with a gimmick tacked on, instead of a fleshed out person. If we're seeing so much of their lives, we should see not only their trait, but how they feel about their trait, what those around them feel about their trait, other traits that interact with that trait, etc.
For instance, if your main character is super heroic and self-sacrificing, never hesitating to put themself in danger to save people they barely know, please dear god explain why they are that way, most real people I see in my everyday life do not behave that way and I need reasons :'D Are they trying to make up for something? Are they trying to prove a point? Do they just not care about themselves for some particular reason? Is this just the kind of person they want to be, and they've went through years of self-reflection and training to overcome their human fears and become this sort of person? What drove them to put all this effort into pursuing this ideal, moreso than other people who aspire to this ideal?
And even if I don't get answers immediately, you gotta drop some hints and give me hope that you've actually thought things through and I'm going to get those answers at some point. Even then, I might not care about your character if what I've seen so far doesn't resonate with me personally, but at that point you're treading into 'everyone's different' territory so it's out of your control
Do the above, and someone will care about your character.