Midna

Midna is a character in The Legend of Zelda: Twilight Princess who she teams up with the protagonist, Link, to battle a "greater evil" that she alone cannot overcome. In fitting with her dark nature, she has a playfully devilish sense of humor. She also has the most extensive voice acting of any character in the game, as all of her dialogue is represented by audible (though unintelligible) vocalization.

Gameplay
Midna is similar to Navi or Tatl. In the Twilight Realm, she rides on Wolf Link's back, helping him with some attacks and jumps. In the normal world, she is but a shadow, yet can still communicate with Link, and is needed to pass into the twilight from the "light" world.

Early in the game, Midna will allow Wolf Link to teleport to twilight portals across Hyrule, thus quickly spanning great distances. Once Link obtains the Master Sword, talking to Midna will allow Link to freely transform into wolf form, and vice versa.

Story
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. In recent time, the Twili have been converted to impish "shadow creatures" by the self-proclaimed Twilight King Zant.

Midna first meets Link when he first strays into the Twilight Realm, and is transformed into a wolf. She helps him escape and return to Hyrule. Midna makes it clear that she is only helping Link to save her own world and has no concern for the Realm of Light, but at a certain point in the game she is gravely injured by Zant and Princess Zelda seemingly sacrifices herself to save her life. Spurred by this selfless act, Midna takes on a much more amiable and less devilish attitude from this point on, and decides that the Realm of Light must be saved as well as and her own Realm of Twilight.

Later in Twilight Princess, it is revealed that Midna is the eponymous "princess" and true ruler of the Twilight Realm. When Zant is defeated, she regains the magic of her inheritance, and is able to use the Fused Shadows to transform into a giant spider-like creature. By the end of the game, Ganondorf's spell is broken, and Midna regains as a beautiful young woman. In a comical moment when Link first sees her true form, she is amused by his silence and asks if her beauty has left him speechless. She then takes leave of the Light Realm, sadly telling Link she'll see him later. Unable to convey any more words, she uses a single tear to shatter the Twilight Mirror that connects the worlds. Midna returns to her realm just as the mirror breaks, and she is never seen again.