1) Look, your MC’s significant other is an important character. As an important character, you want to give them the major character treatment probably even if they aren’t a major character. Which means you want to stretch their spine. Stretching a spine comes from Blake Snyder, don’t hate me. The spine of a character is the change arc they go through. To stretch the spine you exaggerate the difference between how a character is at the beginning of their journey and how they are at the end. A drug using hooker who ends up the next Mother Theresa is a character with a very stretched spine. You don't have to do anything so extreme. But you do want at least one major trait of any major character that pretty much reverses over the course of a story. This isn't giving them a flaw. This is giving them something fundamental about themselves to change because they won't be fulfilled otherwise.
2) Speaking of fulfillment, every character is out to fulfill themselves. Take Belle from Beauty and The Beast as an example. Belle is not in that story to fix the Beast. Yes, that's what people want from her. It's part of Beast’s explicit goals. And she does "fix" him. But at no point is that one of her goals. It's literally never a goal for her. She has her own goals and needs and she spends every scene trying to fulfill those for herself. No amount of flaws or spine stretching would fix her trying to make someone else's goals come to fruition. A character worth reading about is a character with their own ambitions, needs, and drives. You cannot have a 3D character without them. In every single scene they appear in, they are Allowed to help someone else meet their goals, but they MUST be pursuing their own goals. Such things can overlap in stories where one character is trying to change another to be more like what they want, it works at first because they both want to change, until conflict emerges because it becomes clear that the change each wants is different from the other. That's just one example of conflict. You can do anything. But you need Conflict, it's the engine of story and it comes from goals that don't align. The more the goals conflict, the more interesting the story and the characters are to read. The less they conflict, the less power any of it has.
3) Speaking of power, there's the power to effect the emotions of the reader. Do you know any perfect people? I don't. I'm certainly not one myself. So give me even an extremely good person and I'm not going to identify with them. People like to see heroes. They like to see them get built up, knocked down, and see them rise again. It's a thing. But you're watching that, not feeling it. You feel it when it is someone like you or someone close to you. When you IDENTIFY with the character. Identification can be caused by aspiration. I want to be like them. But more often it is the recognition that I am like them. And it's the little flaws and foibles and human frustrations that give that recognition. This is where flaws actually help. It's not to round the character out, it's to make them feel like they could be us. And at some level, most major characters need something like that. Not just any flaw but human flaws, reassurance that they are just like us and are going to do what we would do in these sorts of situations. Which isn't going to be the perfect response. Especially not the most perfectly giving response.
4) Relationships map both ways in fiction. A has a relationship with B. That's your main story. But B also has a relationship with A and while that may not be the main focus, to feel real, B has to be in that relationship just as much as A. Their wants, goals, needs, flaws, all of it have to be a real part. They cannot simply exist in A's relationships and be a fully formed character. If you really want to nail this down, tell the story from B’s point of view. If it doesn't work from their point of view, then the story can't really hold up when it is from A’s point of view either unless your audience is ok with sexy lamps. It must be plain that your MC is giving their significant other what they need or they won't be believable.
5) To sound like I'm contradicting myself, it's also a great idea to show what is draining them in the relationship. What makes them not want it? The real exposition of a 3D character happens in the meeting of their wants/needs/desires vs their fears/repulsions/baser instincts. Three Dimensionality comes in self conflict, in the moments where the character struggles against themselves about whether to change or not. Mostly they choose not to change but the struggle shows their arc and proves they're trying. The kiss of death comes to characters who have no internal or external conflict. They need somewhere they want to be and a reason they can't get there. A person who is good who wants to be good isn't a story because there is no struggle. There are no ideas to struggle with. A bad person who wants to be good works. A good person who wants to be bad works. A faithful spouse who doesn't think they can stand even one more transgression from their once lovable asshole works at the inevitable next screw up. It's the struggle itself that makes things work, not whether a character is good, bad, or indifferent.
(minimally adapted from this reddit thread)
2) Speaking of fulfillment, every character is out to fulfill themselves. Take Belle from Beauty and The Beast as an example. Belle is not in that story to fix the Beast. Yes, that's what people want from her. It's part of Beast’s explicit goals. And she does "fix" him. But at no point is that one of her goals. It's literally never a goal for her. She has her own goals and needs and she spends every scene trying to fulfill those for herself. No amount of flaws or spine stretching would fix her trying to make someone else's goals come to fruition. A character worth reading about is a character with their own ambitions, needs, and drives. You cannot have a 3D character without them. In every single scene they appear in, they are Allowed to help someone else meet their goals, but they MUST be pursuing their own goals. Such things can overlap in stories where one character is trying to change another to be more like what they want, it works at first because they both want to change, until conflict emerges because it becomes clear that the change each wants is different from the other. That's just one example of conflict. You can do anything. But you need Conflict, it's the engine of story and it comes from goals that don't align. The more the goals conflict, the more interesting the story and the characters are to read. The less they conflict, the less power any of it has.
3) Speaking of power, there's the power to effect the emotions of the reader. Do you know any perfect people? I don't. I'm certainly not one myself. So give me even an extremely good person and I'm not going to identify with them. People like to see heroes. They like to see them get built up, knocked down, and see them rise again. It's a thing. But you're watching that, not feeling it. You feel it when it is someone like you or someone close to you. When you IDENTIFY with the character. Identification can be caused by aspiration. I want to be like them. But more often it is the recognition that I am like them. And it's the little flaws and foibles and human frustrations that give that recognition. This is where flaws actually help. It's not to round the character out, it's to make them feel like they could be us. And at some level, most major characters need something like that. Not just any flaw but human flaws, reassurance that they are just like us and are going to do what we would do in these sorts of situations. Which isn't going to be the perfect response. Especially not the most perfectly giving response.
4) Relationships map both ways in fiction. A has a relationship with B. That's your main story. But B also has a relationship with A and while that may not be the main focus, to feel real, B has to be in that relationship just as much as A. Their wants, goals, needs, flaws, all of it have to be a real part. They cannot simply exist in A's relationships and be a fully formed character. If you really want to nail this down, tell the story from B’s point of view. If it doesn't work from their point of view, then the story can't really hold up when it is from A’s point of view either unless your audience is ok with sexy lamps. It must be plain that your MC is giving their significant other what they need or they won't be believable.
5) To sound like I'm contradicting myself, it's also a great idea to show what is draining them in the relationship. What makes them not want it? The real exposition of a 3D character happens in the meeting of their wants/needs/desires vs their fears/repulsions/baser instincts. Three Dimensionality comes in self conflict, in the moments where the character struggles against themselves about whether to change or not. Mostly they choose not to change but the struggle shows their arc and proves they're trying. The kiss of death comes to characters who have no internal or external conflict. They need somewhere they want to be and a reason they can't get there. A person who is good who wants to be good isn't a story because there is no struggle. There are no ideas to struggle with. A bad person who wants to be good works. A good person who wants to be bad works. A faithful spouse who doesn't think they can stand even one more transgression from their once lovable asshole works at the inevitable next screw up. It's the struggle itself that makes things work, not whether a character is good, bad, or indifferent.
(minimally adapted from this reddit thread)