"The more fleshed out the character is, the more the reader will care about him". Always true?

First, congratulations on sticking with it so far. Let's try to fix some stuff.

... the more the reader knows about him, the more they will care about his death, and thus his death being a great loss.

It depends on what they know! If they know he stomps on puppies, they don't care about his death.

As a writer, you should often be thinking about Ramifications. There are a few, with regard to writing in general, but let us consider character development.

Ramifications to the plot: A character trait can influence the plot: Alice loves her striped underwear. When we are searching for Alice, we find striped underwear, cut off somebody with a knife.

Ramifications to the emotions of other characters: How do other characters feel about them? Why do Brad's friends like him? Why does Brad's wife love him? Why does Brad's dog love him? As a writer, don't take such relationships as a given, the good will of friends and lovers and even animals is not permanent.

Say Brad knows and loves dogs. He stops for gas at a corner store, and sees a matted stray, hopping away on three feet, it's front right leg hurts too much to walk on. Brad finishes up with the gas, and parks the car, but has to go look for the dog. He finds it; the lame leg is broken; the dog was apparently hit by a car. Brad immediately calls in late for work.

It takes him an hour to gain the trust of the dog, which Brad names Butch. He brings Butch to his vet, and charges with his Visa the surgery and casting, a full medical examination, vaccinations, and a wash and haircut for Butch. He ends up five hours late to work, missing an important client meeting, which truly pisses off his boss when Brad tells him why (Brad is a truthful man). When Brad gets home he can't wait to tell his wife Alice the story of poor Butch and his broken leg.

Alice interrupts him, "How much?" He thinks she is focused on the wrong thing, but reluctantly tells her: It cost $1800. Alice is distraught, tears well up, "God damn it, Brad. God damn it." She shouts, "Eighteen hundred? Are you insane?"

However, one of the reasons Alice loves Brad is because of his selfless love of dogs. Which she wouldn't take away, but still, he's causing her hardship and grief.

Ramifications to the character self: How do the character traits make this character feel? Why does he do what he does? How do his traits influence his thoughts, and get him the things most humans want? Food, Sex, Shelter, Companionship, Camaraderie, Accomplishment, Success, Money.

How does helping Butch help or harm Brad? Or, how would NOT helping Butch help or harm Brad? Would he have a bigger number in his bank account but be miserable or guilt ridden?

Ramifications to the emotions of the reader: Readers judge the characters. How do you judge Brad in the story above? Do you want Brad as a friend, or nothing to do with Brad? Do you want Brad as an employee? Would you want Brad (or a female version of Brad) as a spouse?

Character traits translate into Emotions that drive actions and actions have ramifications, as a character moves through the world. The traits shape the story, the relationships, the relationship of a character with themselves (what they feel joy about and guilt about), and the plot.

If you are including character traits that have no ramifications, you should not be.

The ramifications to the reader include sympathy and empathy for the ramifications of Brad's death, in Brad's world. What will become of Alice, if Brad takes a risk with his life to save one of his dogs?

It is five years later, and in an epic hurricane Brad's truck stalls in deep water that is still rising. Brad and Butch have to get out, the truck is going to be lost. The water is too deep for Butch to walk and too fast for Butch to swim, so Brad carries Butch through thigh-high water. It knocks him down; Butch is swept into the river. Brad is terrified by the river. He knows he could make it to dry land, he even starts toward it, already crying for Butch --- but he can't do it. He looks to the sky, shouts "Fuck you!" to the hurricane, or God, then turns in anger and wades, then swims, into the swollen river after Butch. Searchers find Brad's lifeless body on the shore a mile downstream, being guarded by a muddy and despondent Butch.

[Let me note here that the entire plot of this two-paragraph story rests on Brad's love for dogs, a character trait revealed in the beginning.]

It isn't exactly about how much they know about Brad. If you want readers to be devastated by Brad's death, then Brad's death must make a difference, in a concrete and literal way: What will be different about this world without Brad in it? In the reader's imagination, what becomes of Butch and Alice?

In other words, what are the ramifications of Brad's death? I've only given two characters, but in a story many can be involved. For the audience, the ramifications should be that the world is a much worse place without Brad in it, so much so it is worth crying for the loss. (On the flip side, the audience will root for the death of a villain because of ramifications: the world is a much better place without the villain, so much so it is worth cheering for the win.)

Now I detect some one-sidedness in your writing about his mother; and I'd caution against that. I would write Brad as human; he has selfish needs too, he can be lazy at work, he can not really give a crap about politics.

Or maybe Brad is not so truthful when it comes to his sales job. His love for dogs does not imply he loves people! Dogs don't ever lie to Brad and to him are easy to understand, control and make happy. So maybe Brad is suspicious and cynical of when it comes to people. Brad has zero tolerance for charities because he thinks they are all scams for some sociopaths to make a quarter million dollars a year while using ten percent of the donations to pretend like they are helping people when they are lining their own pockets. So he doesn't give anybody paid by any charity the benefit of the doubt, and thinks any volunteers are soft-hearted fools being robbed. He has a lot of anger and verbal abuse for anyone paid by charities.

The point is if you make somebody too relentlessly good, the audience cannot identify with them, because 99.999% of us are not relentlessly good! That said, the flaws you give a good character should be understandable by the audience, and forgivable (perhaps with some reluctance) in light of the good that they do. In the end the reader must judge Brad's world as a better place with Brad in it.

You can feel emotions, empathy for those characters with a single short scene,

Precisely.

but with this protagonist, even with a whole story about him, it's not so easy for me.

Probably because the "whole story" is boring, to you and the reader, because the traits you have given him have no ramifications. That makes them a wasted investment in reading. Nobody cares if he likes to read mysteries, if there are no mysteries for him to solve. Nobody cares if he likes strawberry jam better than grape, unless that informs something else in the story.

Another way of saying traits must have ramifications is to say character traits should be connected to the story in some way.

Yet another way of saying this is the idea of "show don't tell," but to me that is too cryptic an advice to give a writer. Because of course, everything written is "telling", I'm telling you what Brad did, what Alice did, and so on.

What it means is, do not tell us about Brad's emotions, do not write that Brad loves dogs to the point of irrationality. If that is your premise then it has ramifications, and you should let the reader figure that out based on the ramifications in the story.

Don't try to come up with clever or poetic wording to convey that trait! A trait must influence Brad's thinking and actions, and that is the "show" part of that maxim. What Brad DOES when meeting Butch, and Brad's other actions and time taken out of his life to care for other dogs, shows us how Brad feels about dogs.

We don't have to be told, and we should not be told: The scenes let the reader develop their own beliefs about Brad. Those will stick with the reader, while a laundry list of traits to memorize will not.

Can you picture Brad? I think you probably have an idea, even though I have never described a single physical trait about him. But in my story (created for this post) it makes no difference what Brad looks like, his age or race or height or weight, his accent. What matters about Brad is his emotional self. If Brad is ugly or handsome, that needs to influence the story.

The audience wants to know more about the main characters, but everything you tell should have some influence on the story. The ramifications of your protagonist's death are not just whether he succeeded or failed, that is worth only a moment of gratitude. The impact is in what the reader knows he has given up, and what others will lose by losing him.

Short answer no, you can get to know a character very well without actually getting invested in them, especially if the character is "a bit of a tool". To get people to care about a given character you need them to relate to that character. The reader should feel that in the same situation as the character they would have done just the same thing. It often helps if the character is also endearing and "likable" as well as relatable but it's not required.

It boils down to "show, don't tell".

You as a writer need to know your characters on a deep level, so you can show the reader who they are through their actions and words.

As you already have discovered through your other characters, the effect is often the greatest when you reveal a lot of character with few words.

If you need to flesh out a lot about a character to the reader, is often a sign that you don't know the character all that well yourself. You need to know them well enough to know what they would naturally say or do in a situation.

Another point is to reveal only information that is relevant to the story. If your protagonist's mother died when she was six (to create sympathy, for instance), but the psychological implication of this has nothing to do with the character's development in the story, then leave it out.