Supporting characters don’t need a full biography, but they shouldn’t feel like props. A few targeted headcanons—one habit, one reaction, one dynamic with the protagonist—can make them feel real without eating your prep time.
One pass per important side character
For each supporting character who appears more than once, run them through the Character Headcanon Generator once. Ask for one thing: "how they react when interrupted," "their tell when they’re lying," or "one mundane habit." Save one or two results. That’s enough to keep them recognizable. You’ll find the generator on the AI Headcanon Generator homepage.
One relationship beat if they pair with the MC
If a side character has a real dynamic with your protagonist—mentor, rival, comic relief—use the Relationship Headcanon Generator for that pair. One micro-ritual or tension (who teases whom, who defers) will make their scenes feel specific. All four tools are linked from the homepage.
Use random for walk-ons or one-scene characters
For a character who appears once or twice, the Random Headcanon Generator can give you a single "suspiciously specific" detail so they don’t feel like a cardboard cutout. The Incorrect Quotes Generator can help if you need one line that sounds like them. Both are on the main site.
You don’t need a bible for every minor character. Hit the AI Headcanon Generator homepage when you introduce someone new, run one or two generations, and drop the best detail into your notes.