Generally, the most important things an author wants to communicate to an audience are the things that are difficult to instantly demonstrate, such as a character's role in the story, personality, moral character, or whether the author wants the audience to sympathize with them.
Often as a visual shorthand, the writer uses the character's appearance to symbolize those characteristics. In animation, a common practice is to use Hair Colors, or the color and shape of the eyes. However, another technique that has several useful advantages is the style and length of a character's hair. For one thing, it's relatively easy to implement in Live-Action TV as well as animation. It's also convenient for characters who undergo lots of character development as you can give them an Expository Hairstyle Change or Important Haircut.
Note: while Hair Tropes are all tropes that have to do with hair, this is specifically a supertrope of cases where hairstyle is used to represent a character's personality, not simply to look interesting or help you tell the characters apart.
Compare Clothing Reflects Personality and Stock Costume Traits, where a character's costume indicates profession & status.
A Super-Trope to:
Female- Boyish Short Hair: Short, unkempt, or spiky hair, often used to indicate a Tomboy and an Action Girl.
- Braids of Action: A single long braid or a pair of braids, used to indicate an Action Girl.
- Childish Bangs: Straight bangs across the forehead with the rest of the hair left long, used to indicate physical or mental immaturity.
- Girlish Pigtails: Hair tied into side tails that stick out the sides indicate youthfulness or immaturity.
- Hairy Girl: A girl with more body hair than considered normal, indicating unattractiveness and/or unkemptness.
- Hime Cut: Long straight hair with full bangs and shorter sidelocks, used to indicate an Ojou and a Yamato Nadeshiko.
- Long Hair Is Feminine: Long hair used to establish the femininity of a female character.
- Motherly Side Plait: A loose braid or ponytail resting on the shoulder to indicate motherhood with a sweet character.
- Ojou Ringlets: Side curls on female characters often indicate royalty or being of (or formerly of) high status.
- Power Hair: Short hair reaching the jawline, parted in front and coiffed expertly so it never crosses in front of the face, used to indicate an Iron Lady.
- Prim and Proper Bun: Hair tied in a single bun at the back, indicating a neat, prim, proper, and organized woman of authority, who is typically conservative and tight-laced.
- Regal Ringlets: Corkscrew curls as a sign of a character being upper class.
- Tomboyish Ponytail: Wearing a ponytail not intended for elegance indicates a Tomboy or The Lad-ette.
Male
- Bald of Evil: Bad guys are often completely bald.
- Barbarian Longhair: Long, unkempt hair as a symbol of wildness or badassery.
- Braids of Barbarism: Horny Vikings and other barbarians.
- Dreadlock Warrior: Dreadlocks are preferred by tough fighters.
- Long-Haired Pretty Boy: Long hair used to emphasize the character's status as a Pretty Boy.
- Sailor's Ponytail: Sailors wear their longish, often unkempt hair in a low ponytail.
Unisex
- Bald Head of Toughness: Baldness (or a shaved head) is used as a visual short hand for physical and mental toughness.
- Bald Mystic: A bald person is mystical or enlightened.
- Bald of Authority: A bald person is associated with authority and occupies a position of authority.
- Delinquent Hair: Unnatural hair color or styles used in a more realistic setting to denote a delinquent or punk.
- Devilish Hair Horns: A hairstyle that looks like horns is usually a sign that the character is up to no good.
- Expository Hairstyle Change: Hairstyle change signals a characterization change.
- Flashy Protagonists, Bland Extras: Protagonists have more eye-catching hair colors and hairstyles than background extras.
- Good Hair, Evil Hair: When this supertrope is used specifically to distinguish heroes and villains.
- Hair Antennae: Two locks of hair that stick out above the head, resembling an insect's antenna, used to indicate The Ditz.
- Hiding Behind Your Bangs: Hair covering one eye, used to indicate a character who is either shy, antisocial, or hiding something.
- Hot-Blooded Sideburns: Sideburns that look like a flame indicate a Hot-Blooded character. On a girl, they usually indicate tomboyishness.
- Idiot Hair: A single lock of hair that sticks straight up, indicating a foolish, naive, or mischievous individual.
- Messy Hair: The go-to style for eccentrics or those who don't care about their looks.
- Nonconformist Dyed Hair: Unnatural hair color used to indicate an alternative or rebellious character.
- Ominous Hair Loss: Loss of hair is a sign of poor health...
- Power Dyes Your Hair: A unusual hair color directly caused by some type of power.
- Prematurely Grey-Haired: Grey hair on a person too young to have it naturally indicates severe trauma in their past.
- Quirky Curls: Characters with completely curly hair (not just Regal Ringlets) tend to be energetic, quirky, rebellious and/or eccentric.
- Samurai Ponytail: This character is nobility, whether they act like it or not, and probably a badass.
- Spiky Hair: A combination of wild, cool, and Badass.
- Sultry Bangs: A character's covering one of their eyes emphasizes their sensuality and confidence.
- Villainous Widow's Peak: A distinct point in the hairline in the center of the forehead indicates a villain.
- Wild Hair: Shaggy, unkempt, long hair, used to symbolize that a character is close to nature.
