Quite difficult, but it depends on the genre of your story and what role the character plays.
To begin with, there are many factors that make a character realistic. But the most effective will be the flaws in character and the realism of his dialogues and actions. Poor execution of a character's dialogue will be similar to that legendary bad scene from star wars, sand. The tone of your light novel is also important. If this is a comedy, then you can make the background characters less realistic, focusing on the main characters.
Now, what makes a character good? For me there are three factors. His theme, character, actions and interactions with others. If the theme of your novel is, for example, war. You can make many characters that reflect this theme in different ways. They may have an ideology opposing the main character or, on the contrary, complement it. Example of Golden Kamui (I don’t remember spelling the name correctly.) It’s not difficult to create a personality for a character. It doesn't have to have any cliches trope, but it's easy enough to make it stand out. And the most important thing for me is the character interactions. It’s not interesting to watch a character who doesn’t talk to anyone throughout the entire story and everything revolves around him and only him. Nor is it interesting to watch a story that revolves only around fights. jjk, the story had potential but instead of characters, it focuses on fights. (I still won’t forgive the author for not letting yuji and Kenjaku talk to each other). If the emphasis in your short story is on the interactions of the characters, their ideology, and motivations. Then you get hunter x hunter. Watch or read it, it's worth the time.
So the bottom line is, to make a good character great, you need to be a great writer. Do what you like, and then you will gain experience. I'm a bad writer myself, but I hope this will somehow help you.