Oh, really, there's no one-size fits all formula. My biggest piece of advice would just be to have a bit of variety in the characters so there's a greater chance that people will be able to find someone who appeals to them. Other than that, making characters distinctive and interesting, with a variety of traits, is good advice for any character.
Or I could just put in the link: https://forum.choiceofgames.com/t/zombie-exodus-safe-haven-live-discussion-feedback-characters/21491/989?u=tssl
It's at the very end of the post, and it's hidden until you click on the arrow.
You certainly wouldn't want to copy these points, by any means; you want your characters to be your own. However, it does show you the kind of detail work that a good author can pack into a character in order to make him memorable.
(And hey, I'm going for a degree in Tommy studies, don't knock it )
No, no, that's just what will inevitably happen if you write something that @ParrotWatcher and I are reading
Trust me, as soon as one of us (probably both of us) like a guy character who you write, you'll know.