Midna, in imp form
Midna, in her true Twili form
Midna is the titular deuteragonist of The Legend of Zelda: The Twilight Princess. She is a resident of the Twilight Realm and a descendant of the ancient Twili people.
Princess 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, Nayru and Farore after an event known as the Interloper War. This alternate dimension, out of which the sublimated descendants could only exist as shadows, eventually became known as the Twilight Realm.
Prior to the events of The Legend of Zelda: The Twilight Princess, Zant, one of the Twili, overthrew Midna from her throne and placed a curse on her, giving her the imp form that she maintains for most of the game and depriving her of the magical powers that are granted to the leader of the Twili. Midna, already in possession of a Fused Shadow, leaves the Twilight Realm and witness Link as a wolf being taken away by a Shadow Beast. Knowing the Twili legend that the Hero would appear as a "divine beast", Midna follows Link to his prison cell in the Castle Town Sewers and tells him that she will help him escape if Link does exactly as he is told. At first, she seems to be a strange, dark creature with an agenda of her own and little regard for Link, but she ends up changing her attitude towards those who are willing to help her. The pair eventually exit the sewers and go to a tower in Hyrule Castle to meet up with Princess Zelda, whom Midna mockingly calls "Twilight Princess", and blames for Hyrule's current state.
After the meeting with Zelda, Midna reminds Link that he must still save Ilia and the rest of the Ordon children that were kidnapped by King Bulblin, saying that she will be happy to help the young hero if he acts like her "servant". Once Link finds the first Fused Shadow in the Forest Temple. Midna reveals that was the item that she was looking for, but refuses to tell Link their exact purpose of what she plans to do with them. The power of the Fused Shadows can be used to destroy Zant, returning Midna and the other Twili to their true form. Midna does not tell Link, however, that she wants the Fused Shadow for the benefit of herself and her kingdom.
Upon collecting all Fused Shadows, Zant confronts Link and Midna in the Spring of the Spirit Lanaryu and easily takes the Fused Shadows from them. He then tries to persuade Midna to join his side, but after she refuses, he holds her up in front of the Light Spirit, Lanaryu, who gravely injures Midna by overexposing her to its light. With Link now once again as a wolf and Midna nearing death, the young hero takes her to Princess Zelda in Hyrule Castle. The dying MIdna, no longer caring for her sake, asks that Zelda tell Link how to break the curse, since that he is the only one who can defeat Zant. As a last request, Midna pleads to Zelda to inform Link where he can find the Mirror of Twilight. Zelda, realizing who Midna really is and touched by her consideration of the world of light, despite her injuries, chooses to revive the Twili by transferring her soul to Midna's body, disappearing in the process. After seeing the selfless acts made by Link and Zelda, has a change of heart and vows to help save Hyrule from Zant. The duo eventually arrive at Gerudo Desert, where Midna asks for Link's help in finding the Mirror of Twilight, their last potential link to Zant.
Eventually, Midna and Link reach the Mirror Chamber and find that the Mirror of Twilight has been broken into four Mirror Shards, with only one of them remaining in the Mirror. After learning the locations of the remaining three Shards from the Sages, the pair find them and return to the Mirror Chamber to complete the Mirror, where the Sages reveal that Midna is the true Twilight Princess and the true 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 that she maintains for most of the game. This curse could presumably not be lifted until Ganondorf, the source of Zant's power, was defeated.
Once that Link has defeated Zant in battle, Midna is shocked to see that her curse is not lifted, which would enable her to regain her powers. Angered, she uses only a fraction of the power that the Fused Shadows hold to destroy Zant, and is utterly bewildered by their power. With the help of her ancestors' magic, Midna transforms into a giant, spider-like beast, with a large pole-arm weapon to break the pyramid-shaped barrier around Hyrule Castle, allowing the duo entry into the Castle hall, where Ganondorf and Zelda are located. Link, after a small confrontation between Midna and Ganondorf, successfully drives Ganondorf out of Zelda's body. However, because Ganondorf's spirit reappears, Midna chooses to sacrifice herself by using the Fused Shadows' power once again in an attempt to destroy Ganondorf. Before doing so, she warps Link and Zelda out of the Castle before she transforms into the spider-like beast, fully intent on killing Ganondorf. Outside of the Castle, Link and Zelda witness an explosion coming from Hyrule Castle, but it is soon revealed that Midna's sacrifice was in vain: shortly thereafter, Ganondorf is seen holding up the helmet part of the Fused Shadows that was Midna's trademark garment, breaking it in his hand to symbolize her defeat.
After Link, with assistance from Zelda, defeats Ganondorf once and for all, the Light Spirits revive Midna and break the curse placed on her by Ganondorf's magic, 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 returning to the Twilight Realm, Midna bids Link and Princess Zelda farewell in the Mirror Chamber and tells them to remember that there is always another world bound to the one that currently exists. She then addresses the young hero with "I... see you later." She sheds a single tear which, upon contact with the Mirror of Twilight, destroys it completely, severing the only link between the two worlds. She quickly runs to the platform to be taken back to the Twilight Realm, smiling at Link as she does so, before disappearing into her world completely.
In Hyrule Warriors, Hyrule Warriors Legends, Hyrule Warriors: Definitive Edition, Midna appears as a playable Warrior. She uses the Shackle and the Mirror as her Weapon, with which she uses to conjure powerful Twilight attacks. Her fighting style is brutal and wild, often consisting of her grabbing and throwing her enemies, using her hair as well as summoning Shadow Wolves to attack. One of Midna's attacks transforms her into her Fused Shadow form from The Legend of Zelda: The Twilight Princess.
And as her true form, she attacks using the Mirror, a Weapon based on the Mirror of Twilight. The Mirror is influenced by Midna's power to warp objects in The Legend of Zelda: The Twilight Princess, such as her own Zant's Hand for melee attacks, Sols, Sky Cannon Link used to reach the City in the Sky to fire Sols, the Bridge of Kakariko Gorge and Twili Wolves that appear gigantic, compared to her Imp form. Her Focus Spirit is an reenactment of the ending of The Legend of Zelda: The Twilight Princess, where she shattered the Mirror of Twilight, though in this case, she apparently shatters reality itself.
As Twili Midna, she is also a playable Warrior in Hyrule Warriors, Hyrule Warriors Legends, Hyrule Warriors: Definitive Edition, first appearing in the Twilight Princess pack DLC. She is the true form of Midna from The Legend of Zelda: The Twilight Princess.
In the game's Legend Mode, Cia is said to have transformed Midna into her imp for, and she is now searching for Cia in order to be changed back. She is first encountered as a villain by Lana and Agitha in The Land of Twilight, having stolen Agitha's Goddess Butterfly after learning it could guide her to Cia. She is soon tracked down by Lana and defeated, conceding defeat and surrending the butterfly. However, after learning that the Hyrulean Army are not aided by Cia, she willingly joins their cuase and travels with them to the Palace of Twilight, where the era's Gate of Soul is being guarded by Zant.
In her true form, she appears in the Her True Self scenario of Linkle's Tale. At some point after Link received the legendary Master Sword while fighitng Wizzro, Zelda and Midna leave their group to wander off to the Palace of Twilight, upon hearing a "mysterious darkness" has entered the domain. Coincidentally, Linkle happens to get lost there; the young villager suddenly finds a magical black crystal that so happens to restore the imp Midna to her true form.
In the Legacy of Chris Thorndyke, she plays a major in the Twilight Princess arc of the sixth saga, that she accompanies Link in his battle against his Ganondorf from his timeline.
And she is voiced by Laura Bailey in imp form, and Cree Summer in true form in the English dub, but reprised by Akiko Komoto in the Japanese dub.