Hair is a funny thing. It's slightly different for everyone; the colour, the texture, the smell. After that, it's styled, dyed, chopped, and changed again. Hair is always changing for a person.
Because of that, it can be useful to use hair to describe character growth. A sporty person will tie their hair back or chop it off, and might have a greasier, sweatier texture. A popular person will load their hair with product and maybe iron it straight. Everyone is going to treat their hair slightly different.
What is your protagonist's hair like? Your antagonist? The best friend? The love interest? Describe some hair--either what it is, or what the character wants it to be. Hair can be revealing.