Characters are the people who populate your book. From the protagonist to the horrible bad guy to the cute little girl next door who listens to the neighbors and learns exactly who is sleeping with whom. I've known many writers who say that all of their characters are actually them. And that likely is true to a great extent. However, I have never killed anyone in reality. Only in fiction. I try very hard to make that character unlikable enough that someone wants to kill him or her. You don't have to write much about the dead character if you'd rather not. But you might want to let the reader see who that person is through the eyes of the other people in your book. Especially the characters who might have the best reason to kill that person. And you hope there is one person who has the best reason. And the means and opportunity.
Your good guy or protagonist should be someone you like and you like to spend time with because you might even write more than one story or book with that character. Most of us think the main character is based on our self in some way. But as Sue Grafton says about Kinsey, she's smarter, younger, prettier, slimmer that I am. I'd want my main female character to be that and more fascinating, funnier, and taller than I am. I'd want my main male character to be witty, sexy, good-looking, stronger, smarter and have a better body than my significant other.
If at all possible, you will people your story with other or even minor characters that at least make their presence known to you and to the reader. Somehow it helps if you can get help from your secondary characters to guide your protagonist. Certainly applies to a sidekick character. That person needs listen when necessary, argue with the protagonist if needed or cheer when something makes sense to both of you.
How do you come up with such characters? Beats me. I think everyone does it differently. The main thing with me as far as a protagonist is character that talks to me. The conversation usually involves another character. A sidekick or friend but sometimes even the bad guy. These conversations usually lead to a story or a novel. The characters reveal themselves as I write and listen to the conversations.
Many writers list their character and write extensive biographies for them. Early on I cut out magazine pictures of people that looked somewhat like my characters. I tried to list likes and dislikes. Everyone has a special way to create people for their stories and books. Whatever works best for you is the best way for you.
I love what I do and I love that I can admit to listening to the voices in my head and not feel that the little men in white coats will come after me and take me screaming off to the funny farm. Like Larry Block said and titled one of his books, Telling Lies for and Profit. That's my favorite line.