Twili

The Twili (called the People of the Twilight in Japanese) were a race of people who resided in the Twilight Realm.

History
The Twili were descendants of sorcerers only known as the Dark Interlopers, who attempted to seize control of the Sacred Realm after the creation of the Triforce. They were seized by the three Goddesses, Din, Nayru, and Farore, with the aid of the Light Spirits, and banished to a limbo known as the Twilight Realm. Over a long period of time, because of the vastly different environment within the Twilight Realm, the interlopers evolved from their original forms to a unique, black and white, gentle-looking race, namely the Twili. Eventually they gave up their will to rule over the Sacred Realm and Hyrule and became peacefully adapted to their new lifestyle in the Twilight Realm.

At one point in their history, Ganondorf was banished to the Twilight Realm. There he met the young servant of the Twili Royal Family and nominee for King, Zant, and gave him the power to take over the Twilight Realm and Hyrule. Zant usurped the throne of Midna, the Twilight Princess at the time, transformed his fellow Twili into Shadow Beasts, and invaded Hyrule. He was stopped by Link, the hero chosen by the goddesses, along with Midna and returned the remaining Twili to their previous forms. Midna then left for the Twilight Realm and shattered the Mirror of Twilight, the only known path between Hyrule and the Realm, isolating the Twili forever.

Biology
A Twili's skin was mostly black while their faces, necks, and chests tend to have gray patterns in a marked contrast to the rest of their bodies. The Twili had a stretched appearance, with long limbs, necks, and heads, and great variances in overall height and girth. It also appeared that common Twili were incapable of speaking the Hylian language and appeared to only be able to utter a moan when Link drew near, which may have been their own language. Zant and Midna were notably different in behavior and appearance to the other Twili, though Midna was cursed to an imp-like form throughout most of Link's adventure.

As the few Twili in the game who displayed the above mentioned form reverted to that form (having been originally non-attacking Shadow Beasts) after being exposed to the light of a Sol, it is possible that this is not their true form, but the closest that they can come to it whilst Zant and Ganondorf are still alive. In other words, the Sol had power enough to weaken the evil magic used to transform them, but not enough to break it entirely. This can be seen as one reason why, after being "cleansed" with the Sol's light, they still simply stand around and stare blankly into space, not reacting to Link's presence other than to softly moan (the noise they make is the same produced by Zant during the cut-scene after the battle with him). However, since they were turned into Shadow Beasts, and Link destroyed every Shadow Beast in the Realm of Light, it could be possible that Midna and the few Twili you see in the Twilight Realm (only six were seen.) are the only Twili left. However, the presence of many other floating structures within the Twilight Realm may suggest otherwise. The Twili and the Shadow Beasts are the only creatures that can enter areas of Twilight or bring anything into it.

Advancements
The Twili appeared to possess many strange technologies that were more advanced than that of the Hylians, and even more advanced than technology from Termina. One example would be their use of teleporting across locations using Twilight Portals. In the Twilight Realm, the Twili use platforms for transportation that appeared from nowhere and were completely transparent, except for the glowing green shapes seen all around anything that is of Twilight origin (portals, Sols, etc.). Another example were the doors that open by themselves. These doors just had one green shape when inactive but when one opened them the green shape extended making more details around the door. Another, more likely possibility, however, is that the many wonders of the Twilight Realm are caused by the magic of their ancestors put to a more peaceful use.

Government
The Twili were ruled by a single person by the title of "Twilight King" or "Twilight Princess", depending on the gender. Despite these they appeared to be a democracy, since the King or Princess are elected by the people. The ruler resided in the Palace of Twilight.

Speculative Relations
Note: this is all speculation, and no connection have been made between Twili and any other races by Nintendo themselves.

Sheikah
The main similarities are with the Sheikah, as both are described as wielders of Dark magic, have red irises and dwell in the shadows (even being referred to as 'the shadow folk'). Also the symbol of the Sheikah, a stylized eye with a tear drop, is present on both Zant's throne and on the back of the Fused Shadows.

The Legend of Zelda: Majora's Mask
Another race, though not featured in the series and is only briefly mentioned in The Legend of Zelda: Majora's Mask, are those that used Majora's Mask in their 'hexing rituals'. There are many similarites between the Twili and Majora's Mask itself, which implies that the Twili could be the same race. The similarities involve some shapes featured on Majora's Mask being present on Midna's helmet, the Fused Shadow (the eye especially). Zant's behavior is very similar to creatures specific to Majora's Mask and the Mask itself, such as Zant screeching in a similar fashion to Majora's Mask, being a sympathetic villain much like Skull Kid who wore the Mask. During the mention of those who used the Mask, the Happy Mask Salesman mentions that the mask was hidden in shadows, which the Twilight Realm is sometimes called (to be precise, "The Realm of Shadows"). Also, the entrance to the Stone Tower Temple in Majora's Mask appears to be a large face with it's tongue sticking out. Zant's helmet also resembles a face with it's tongue out, which would lead to the belief that the ones who built the tower (believed to be the same race as the ones who made the mask) could very well be the Twili (also connecting to the Garo, the protectors of Ikana Canyon and the Stone Tower).

Gerudo
Another possibility is that at least a portion of the Gerudo tribe were the ones who gained the mysterious power of the Fused Shadows and were cast into the Twilight Realm. This would be supported by Midna's story that the Twili race were originally a tribe of thieves, and why the entrance to the Twilight Realm is located in the Gerudo Desert. The Gerudo are no longer present in Twilight Princess, and just before Zant is fought within the Palace of Twilight, Midna mentions that the Twili's ancestors "lost their king to greed" - implying that she means Ganondorf, whose desire to posess the Triforce eventually led to his defeat. As for appearance, Midna does have the classic red-orange hair of the Gerudo and her true form is tall and slender, just as the Gerudo women once were. Also, there is a Gerudo pattern and converted Gerudo symbol on the front of Zant's robe, which he wore even before his encounter with Ganondorf, whom he worshiped as a god. Gerudo also worship Ganondorf as a god, which a gossip stone tells you. It is also striking that Veran, who is called a Gerudo Sorceress, is nearly identical in design to Midna.

Zuna
The Twili bear a strong resemblance to the Zuna, and even dress in a similar fashion. Due to the Mirror of Twilight's desert location, and the Zuna's desert home, they seem to have at least some relation. The ancestors of the Zuna were also skilled in dark magic, as they created the Trident of Power. It is possible that the Dark Interlopers were not only ancestors of the Twili, but of the Zuna as well.