It's well known that the eyes are the "windows to the soul" — in fact, it's mentioned in perhaps half of the Eye Tropes we have. What does that actually mean? The suggestion is that if you stare into someone's eyes deep enough, you can glimpse their true nature.
Looking into one's eyes can give one insight into what the person is thinking. At the very least, it can betray some level of emotion, and some even believe one can tell truth from lies, and determine the true nature of one's personality, simply by employing this method. On occasion, the things that we're told can be seen in someone's eyes can get really subtle or really abstract - from sadness or murderous fury all the way to humanity or the lack thereof or immense age.
In the world of fiction, matters go beyond merely distinguishing lies and feelings. Some tales, especially fantasy, associate special traits to the eyes which tell other characters and, more importantly, the reader more about the character who bears them. Most Eye Tropes, ranging from Technicolor Eyes to the likes of Eyes Out of Sight, serve in this capacity, if primarily on a metaphorical level.
This sentiment also underlies many glasses tropes- characters with something to hide may employ Opaque Lenses, Scary Shiny Glasses, and Sinister Shades to hide (or hint at) their true nature.
This broad concept covers many tropes, but the commonality is that eyes signify a character's true nature or self, whatever that may be.
- Color-Coded Eyes: Color symbolism in the eyes used for characterization.
- Dreary Half-Lidded Eyes: The trademark half-closed eyes of the tired or sarcastic character.
- Eye Colour Change: Eyes change color in response to a significant change.
- Eye Contact as Proof: Demand that someone look you in the eyes and state with no equivocation that they feel a certain way about an issue.
- Eyes Are Mental: A shapeshifter who can't change their eyes.
- Eyes Never Lie: The truth is told by looking into someone's eyes.
- Family Eye Resemblance: When an entire family has a distinctive eye color or eye shape.
- Good Eyes, Evil Eyes: Character Alignment depends on how your eyes dilate.
- Quivering Eyes: A character's eyes, or just the reflected highlights in them, shaking perceptibly during times of emotional duress.
- Tareme Eyes: Eyes that droop or sag at the corners, which symbolize soft emotions such as sympathy, kindness, or docility.
- Tsurime Eyes: Eyes that slant upwards, which symbolize strong emotions such as arrogance, pride, or sarcasm.
- Wingding Eyes: A character's eyes become symbols representing their current state of mind.