Kubo-sensei tweeted the cutest stuff about being at a cocktail party the other day and mentioned how he doesn't know how he comes up with his characters. Translation by mezzo_marinaio :
Whenever I go to a drinking party and meet some new faces, without fail I'm asked "How do you create your characters?"... but there hasn't been a single instance where I was able to answer that question skillfully. That's because I myself don't understand exactly how I create them. I mean, I've heard stories... and I'm surprised that there are so many people who fall right into a pattern while creating their characters. I can't do things that skillfully.
On the subject of characters... I think the most accurate representation would be if I answered "I don't really understand it myself... but before I know it, they've been created". However, even with an answer like that I get "??" faces as a response. So I say something similar to that, something that would be more understandable - you know, some sort of mangaka answer. And each time, I say that and run away. Sorry, everyone.