The old Unearthed Arcana Mystic class got dropped, and for good reason. But it did have one nice thing: an inspiring table of quirks. Here it is.

d20Quirk
1You never cut your hair.
2You refuse to wear clothes of a specific color.
3You never say your name.
4You never wear footwear.
5You always wear a mask.
6You dye your hair bright blue or green.
7You pick a new name each day. This one is a nightmare. Pick something else.
8You never immerse yourself in water.
9You sleep on bare earth.
10You never consume alcohol.
11You wear a veil to conceal your face.
12You always wear a specific piece of clothing.
13You refuse to light fires.
14You refuse to write things down, instead using pictograms.
15You never sit on a chair, preferring to stand or sit on the floor.
16You never answer to any name but your own.
17You write down the name of each creature you slay, and name ones that are unnamed.
18You consume only water and raw vegetables.
19You spend any money you earn within 1 week of gaining it.
20You often speak to an imaginary companion, and act only with its blessing.