As with all my great labors these days, I turn to the Internet. And seeking, I found several great essays on character-building, especially my favorite part of it, character quirks.
Here's Writing Geekery on character quirks.
The essential point is that a good character quick blends characterization and character development.
And here's a character quirk I used that didn't: a gold pen. My character originally used it to aid others in time travel. But why? Someone gave this tour guide a gold pen in gratitude, and so the pen had the ability to fold time, or write paragraphs of history. Not a good quirk and certainly not a good logical rule for time travel. I wound up keeping the gold pen -- evidence that this character was wealthy and beloved by his wealthy clients, but not more than than. A minor, not developmental quirk, but then this character didn't develop through my story. He helped others do so, but he didn't have an arc.
It's possible to get so enamored of a character's quirk that you try to build a story around it, and that's fine, as long as it does support a story. But I do love character quirks and how they aid my remembering a character as I read. Sometimes it's an event in a character's life. How she once cut school to go catch crayfish in a creek and instead caught an unopened letter.
And I really love gold pens. Maybe for the next time travel story?