What's a good fault in a character?

They confuse love with lust.

They are a sex addict.

Their parents were killed when they were 10 and they escaped their pain by having sex from a young age. They use it as an outlet for their need to feel "love" or emotion or commitment from other people. By being either a slut or a douchebag womanizer.

Then they try to get married to a "nice" person. Then they are separated by some bad event and are alone for a while a few weeks or months before their weeding. Then one will meet someone nice and mourn their wasted youth and past mistakes. They can't have this person because they're engaged to be married soon... but they lie and sleep with them and fall into a deeper love that's actually Real.

Then they realize their fiancé is a bad person and have to take a stand and fix their life.

Practically a whole story there. Idk. I just pulled that out of my butt there. Sounds kind of emotionally charged right? But it's not supernatural or Sci-fi or fantasy or anything. But it could be if you wanted it to be. Just start writing and see what happens to the characters you invent. Wing it! Then build it into something better.

By "good' fault, I presume you mean a "good example" of a fault rather than a fault that that is actually good to have in some respect.

Suggestions:

1. Holding grudges for perceived slights/unforgiving

2. Unrealistic expectations for self/obsessive striving for perfection

3. Pride/selfishness

4. Gullibility/easily duped

5. Given to excesses or extremes/deviating from moderation (overindulgences/addictions)

Quirks

Obsessively washing hands

A germ a phobe

Afraid to leave the house

Drives old vintage car

dresses slovenly. sharp, goth, preppy

Anything that makes them different and interesting