The characters that have the most staying power seem to be the ones where I can get committed to telling their stories in full. I tend to have a goal in mind for them to get to eventually, and then I can't really let go of them till they get there. If I don't have that, they tend to fade out more.

And honestly having some kind of schedule for when they're doing things has honestly been a huge help in making sure they stay active and keep getting connections to other characters.