Midna

Midna is a character that has been confirmed to exist in the newest Zelda game, The Legend of Zelda: Twilight Princess, which is expected to the released April (2006). Midna is a resident of the Twilight Realm, a dark presence that has spread all across Hyrule. When Link enters the twilight, he transforms into a wolf and is afterwards captured. In his prison cell he meets Midna, a strange, dark creature with an agenda of her own. Midna rescuses Link, and can be seen later riding him in his wolf form while in the twilight.

While Midna did rescue Link, that doesn't mean that she is on your side. It has been said that Midna is evil, but her plans go against the twilight and its residents, so she teams up with Link. We aren't exactly sure how she helps Link in his adventure at this point, but it appears that she can use her powers to help Link target more than one enemy at once and to see things that are hidden. It has also been commented that Midna does exist outside of the Twilight Realm, but only in the most basic form of existance. What could this mean? Only time will tell.

Theories
There are many rumors going around about whom Midna is.

Majora Theory
Based on the "most basic form of existence" quote, some think Midna is Majora. Let's look at the evidence.
 * 1) Aonuma may bring some of his old characters back
 * 2) Majora's voice is a bit girly, when it turn into Majora's Wrath, the eyes of the mask look like breasts
 * 3) Look at the eye of Midna's helmet, then the eye of Majora's Mask. They are very similar.
 * 4) A mask is a pretty basic form of existence

Other Theories
Some think Midna may be Navi. Unfortunately, there is no evidence, but hopefully Midna is not Zelda. How predictable that would be! Whether Midna is a mask, fairy, princess, essence of the Triforce, or something else; at least it's in the masterful hands of Nintendo.