I'm a "Forever Dungeon Master" and as such, have had to develop a knack for coming up with a character on the fly and then retrofitting them with a life and backstory when players decide to delve into it.
Most of my comics, characters have only as much backstory nailed down as is absolutely necessary. Some of them don't even have names!
Errant is the exception, because I made those characters originally in my teens, when I didn't churn out nearly so much content and thought the inner lives and characteristics of my characters were really interesting and important, unlike current me, who cares more about using those characters more as a device to tell a story. So the cast of Errant, I know a lot of detail about their lives, like for example. Rekki's full name is Elrekia Susan Lune, she's a Scorpio, she's left handed and slightly long-sighted so needs reading glasses, and her best subject in school was Maths. This is actually helpful for a story like Errant, because it's got this big ten year timeskip that we keep dipping into to flesh out the story of what happened. Knowing so much about who the characters are makes creating what Rekki's bedroom looked like when she was fourteen relatively straightforward:

I also had to make a timeline generally, for things like, "When did Rekki's grandmother end the Fourth Age?" (answer: When Rekki's mum, Juniper, was about five), and that informs things like on this photo, everyone's kind of 1970s-ish, and Sylvia is about 25 years old, in contrast to being an older lady in the "present".

(You don't even want to know how much I know about Rekki's mum, Juniper Lune, who will probably never be more than a minor background character... oh my god....)
How much do I think about backstory? Usually not so much, but in Errant, way too much 