Twili

"They were banished. They were chased across the sacred lands of Hyrule and driven into another realm by the goddesses."

- Midna The Twili are a race of beings who inhabit the Twilight Realm. Most of the inhabitants are turned into "monsters" under Zant's evil rule. Unlike the other Twili as shown in the game, Midna and Zant are the only Twili who retain their true appearance. All other Twili that are briefly seen in  Twilight Princess were quite different in appearance. It is possible that Zant's curse on the Twili had not been completely lifted at the time Link met them. This is a topic that has been debated among fans ever since; though the Twili look different compared to Midna and Zant, Midna mentioned herself that the Twili had been changed back to their original form.

Clothing
A therory has been created the the dark patches on the Twili's body are not pigmentation as they appear to be, but what the Twili where for clothes: shadows. Because fabric would probally be scarce in the Twilight Realm, Midna and Zant, being royalty, are the only Twili shown to where fabric. Midna wears a simple cape and skirt while the rest of her body is covered in dark patches; Zant due to his vain nature wears a complete outfit covering up his entire body. All other Twili wear no fabric and have dark patches of skin in the shape a Toga with a hood.

Background
Eons ago, the Twili were a group of dark sorcerers, ethnicities unknown, that were sent to the Twilight Realm by the Sages of Hyrule as punishment for their misgoverned attempt to use old magic to claim the Triforce. Their "old magic" became known as the Fused Shadows, a powerful instrument, however, their powers proved too weak for the power of the gods. They were banished and their Fused Shadow was split into four pieces and scattered throughout the two realms, The Twilight realm and Hyrule. This tribe lived in the Twilight Realm for centuries until evolving, mere shadows of what they once were, the Twili tribe became used to their surroundings and came to accept their fate. While some came to love the beautiful peace that can be found only in Twilight, it drove some of it's inhabitants mad. After centuries of rule, Princess Midna became just one of many rulers in a long line of royalty, the Twilight Princess. It was around this time that a rebel force grew from within the Twilight, spearheaded by Zant. It is revealed that Zant was next to become Twilight Ruler, but the position was passed over. In a terrible fit of rage, he used the power of his false-god Ganondorf to transform most of the inhabitants of Twilight into monsters, and as a form of humiliation, transformed Midna into an imp creature. Midna's only hope was to find the magic of her ancestors, the Fused Shadows, and use it against Zant to save the realm and her people.  The Twili in  Twilight Princess are usually seen as enemies, however when Link goes to the Palace of Twilight, he reverts several of the Twili back to their true forms. The Twili have strange glowing markings on their skins, which are a blue/grey tinge mixed with black, this complexion may be due to lack of full light as the Twili have changed from what they were when they were ejected from the "World of Light". They also have oval, yellow eyes and are (usually) quite thin and lanky as well as much taller than Hylians. Midna stood no less than than a full head above Link.

Midna's true form seems more human-like than this as she has distinct eyes and has many other different characteristics that differ from the Twili. This could be due to her lineage as there may be a separate sub-species of Twili in the Royal Family that have more human-like qualities in them, or at least females look more human-like, since Midna is the only known female in Twilight Princess. It is also quite possible that once Ganondorf is killed, and Zant's spirit dies, that the Twili race as a whole are turned back into beings more resembling Midna and Zant.

Timeline, Ocarina and Majora's Mask
The events described in Twilight Princess are likely to have occurred before the events of Ocarina of Time, which is generally believed to be near the beginning of the timeline in Zelda game. This is because the Twili's ancestors' are described as trying to assert dominion over the Sacred Realm before the Triforce had ever been successfully found, and as the Triforce has been split by the time of Twilight Princess, claimed as being several decades after Ocarina of Time, it is likely that it was found during Ocarina of Time. Furthermore, the Twili's ancestors are described as having been imprisoned long ago, long enough for their race to lose all passion and ambition.

Though it has been theorized, Twilight Princess makes no mention of Majora's Mask. Debates arose when similarity between the eye-design of Midna and Majora's Mask were alleged by a number of fans, and now some players choose to speculate that the Twili are in fact responsible for creating Majora in a separate attempt to obtain the Triforce. However, this is contradictory, as the Twili were imprisoned by the Light Spirits for abusing their magic, while the ancient tribe that used Majora's Mask chose to seal it themselves.

Others still believe that the Twili are ancestors or descendants of the Zuna Tribe seen in Four Swords Adventures.

Also debated among fans is if Veran, the antagonist of Oracle of Ages, is of Twili descent.