Midna

Midna is a character in the newest Zelda game, The Legend of Zelda: Twilight Princess. She is a resident of the Twilight Realm and a descendant of the ancient Twili people. At first she seems to be a strange, dark creature with an agenda of her own, but ends up rescuing Link, and can be seen later riding him in his wolf form while in the Twilight Realm.

Midna is one of the Twili, a race descended from wielders of powerful and evil magic who were banished to an alternate dimension by the goddesses Din, Farore, and Nayru. This alternate dimension, out of which the sublimated descendants could only exist as shadows, eventually became known as the Twilight Realm. She remains as an imp-like creature through most of the game. In recent time, the Twili have been converted to fearsome "shadow beasts" and Midna into an imp-like form by the self-styled Twilight King Zant. Midna reveals this information to Link in the Gerudo Desert.

At first, Midna is a "servant" of Zelda, proclaimed as the Twilight Princess. She finds Link, in wolf form, locked in a prison cell, and aids him in his escape. With this comes the new ability to talk to Midna whenever the player chooses, by pressing "up" on the directional pad. Even further along in the game, pressing "up" allows the player to choose from several of Midna's abilities, such as teleporting via portals throughout Hyrule or transforming into Wolf Link on demand.

Midna can be compared to the likes of Navi,The King of Red Lions and Ezlo, in that she provides information throughout the game that serves to aid the player.

As she is turned to her imp form by Zant, it might be an homage to Majora's Mask in which Kafei is turned into his kid form by the antagonist.

After the Twilight Mirror is rebuilt, the Sages revealed much later in the game that Midna is the true Twilight Princess and ruler of the Twili race, not Zelda, who had previously been defined as the Twilight Princess by Midna herself, presumably to keep a cover on her identity. Zant, after usurping the throne from Midna, placed a curse on her that gave her the form she maintains for most of the game. This curse could presumably not be lifted until Ganondorf, who Zant refers to as his god, is defeated.  After Link has faced Zant in battle, Midna uses the power of the Fused Shadows to destroy Zant (temporarily, as he could be revived by Ganon). She uses only a fraction of the power that the Fused Shadows hold to do this, and is utterly bewildered by their power. The Fused Shadow's power is also used to break the pyramid-shaped barrier around Hyrule Castle. Midna uses the power to transform into a giant, spider-like beast with a large pole-arm weapon (see below). This same form is used in an attempt to destroy Ganondorf during the ending sequences of the game, but the attack fails, and Ganondorf supposedly kills Midna, holding up the helmet part of the Fused Shadows that was Midna's trademark garment. 

After Link, with some assistance from Zelda, defeats Ganondorf once and for all, it turns out Midna survived and Ganondorf's spell on her was broken, allowing Midna to regain her true form. In a comical moment when Link first sees her true form, she is amused by his silence and asks, "Am I so beautiful that you've no words left?"

Before going back to the Twilight Realm, she bids everyone farewell and sheds a single tear, which is used to destroy the Twilight Mirror just after she returns to the Twilight Realm. She vows Link that she will "see [him] later," however this is highly unlikely as the Mirror was the last link to the Twilight Realm. As another last word, she asks that everyone "...remember that there is another world bound to this one."

During this ending sequence, as her tear begins to float toward the Mirror of Twilight, Midna addresses Link, beginning with the word "I..." but never finishes her sentence. It is rumored that Midna was interested in Link on a level higher than friendship. This is somewhat evidenced by a few subtle gestures throughout the game, and how she sometimes behaved around Link. Whether or not this is true remains uncertain. 

Midna as the Twili Leader
In the game, after hearing Zelda's story about the Twilight Realm and how it came to be in Hyrule, Zelda says to Midna, "The shadow beasts have been looking everywhere for you. Why is this?" Midna, probably not wanting to reveal her identity, replies, "Why indeed? You tell me!" She is the only one of the Twili who has not been turned over to Zant in the course of the game, as Link finds out. Twilight Princess players may find this confusing and hard to believe. Why would an imp be the only one to escape Zant's grasp? The entire Twili race was caught, so why not Midna?

Midna is revealed to have been touched by not only Zant, but Ganon as well. This is only because of her true identity as the Twilight Princess. Zant the Usurper King of the Twilight Realm is a pretender. As Midna says in the game,"A fake is a fake, and no matter how much you dress it up, the real thing always wins."