Major non-protagonist characters have no reason to be anything other than a whole character, completely formed and thought out. While this does include Antagonists, it mostly is referencing characters that are bad or neutral, but maybe not so bad. As an example, let’s take Lucius Malfoy from Harry Potter: he is a baddie, but he’s not Voldemort. Lots of creators forget to think or put specific traits into their writing when covering these major characters, and that leaves the character with holes in it. So what are these traits that people are forgetting about when forming these types of characters?
Character Development: everyone forgets to give these characters any development at all. You might not see the development in Lucius, necessarily, but you see it in his wife Narcissa and son Draco. Is this close enough? Not really, but I’ll take it. At least those two got some action and became a little different than what they were at the start. How do you give them character development? Unless it’s a plot point, there’s no reason to make their development a huge deal. If suddenly they change sides and become a protagonist in your story, that’s relevant to the plot. If they begin questioning orders from the top baddie, well it might be relevant or it might be them gaining a bit of a conscience. It doesn’t always have to lead to something. Maybe they gained a skill, or some knowledge. That is character development.
Flaws: Flaws are inherent in everyone. It’s important to keep them in mind. Some people have more, some people have less. For some it’s physical, for others it’s mental. Flaws help create a well-rounded character, so don’t forget them!
Opinions: There are always varying opinions out there. Especially over other people. So give these characters their own opinions, arrived at by their own experiences and logic processes. Everyone has opinions, don’t forget them just because these characters aren’t the main antagonist. This includes world views, politics and moralities. Some people are willing to kill, others are not even if they are a baddie (Draco Malfoy, for instance).
Varying Builds: This has to do with physical differences just as much as emotional and mental. Some characters are bound to be less able-bodied or able-minded than others. Look around you in the world, analyze people’s faces as you walk by them. See different gender identities, races, sexual orientations. Cherish each and every one of them. They are all valid character builds.
Traits Solely Their Own: Creators often forget that not all traits shown of a major character have to relate to either the main antagonist or main protagonist. They are allowed to have feelings, expressions as well as likes and dislikes all their own. Maybe they like sugar cookies better than chocolate chip cookies—and it has nothing to do with anything except that it’s part of their character.
The goal here is to have well-rounded characters no matter how large or small their role is. They should each be as unique and easily differentiated from one another as people are in your life: they may share certain traits, but other parts of them will be mismatched. Once you have every character established as a whole character in their own right your world will seem a lot more believable and immersing.