Hylia (Deity)

Hylia (pronounced /ˈhаɪliː.ə/  HY -lee-ə), is the deity after which the Kingdom and world of Hyrule are both named. She is the Goddess of Time, the creator of Fi, the presumed swordsmith of the Goddess Sword, and the sealer of Demise. According to Fi, she exists, watching over Link from "the edge of time," even as she is incarnate in Zelda; although Zelda had not yet completed her reawakening at the time of said quote.

Identifying features
Although she is never seen in-game; she is depicted in the intro, caricatures on walls, and her larger-than-life statue. Several things can be gathered about her namesake, the Tit Hylia, also known as the smallest bird in Africa, and evidence in Skyward Sword:
 * She had wings like an angel or bird, as seen in all her depictions, and possibly alluded by the idea that Loftwing are the other "halves" of the her Skyloft servants.
 * In place of hair, she a mane of feathers, seen on Statue of the Goddess.
 * A large nose, as inherited by Zelda. The only characters depicted with such a long nose are the Sky Person's, or Gerudo who also have an avian influence.
 * She wore a white robe, inferrable from the similarity of her appearance and Zelda's symbolic dress.
 * She was gigantic; evident from the enormity of the Statue, the introduction sequence, and the fact that the ceremonial harp is only a replica of her's.
 * She was beautiful; such that Demise scorns at her having shed her divine grace to incarnate as Zelda.

Personality
Little to nothing is known about her personality, than what can be gleamed in Zelda, who claims to be at the same time, her own person and the Goddess incarnate by Skyward Sword's end. However, that Zelda brings up her theory that the Triforce's edict to forbid a god's wish, was so mankind could have hope for itself, connotes the Goddesses may have planned for there to be a time when deities no longer existed. This paints her as a masterful tactician to devise the destruction of Demise, but a servant obedient of the Old Gods, faithful to a fault.

References or mentions before Majora's Mask
She is first mentioned by name in A Link to the Past, although indirectly in the context "royalty of Hylia" to refer to the Royal Family. Although she is never mentioned in Ocarina of Time, there are various reasons to assume the backstory of Zelda was in planning. These include the following:
 * Zelda's Lullaby has the ability to revert objects back in time, as it is used on Link to send him to the past. It's notable because the Song of Time also has a similar special effect when creating or dissipating Time Cubes.
 * A retroactive connection to the above point; the Ballad of the Goddess in Skyward Sword is Zelda's Lullaby backwards.
 * Zelda's Sage element is never mentioned. Each Sage corresponds to the name of a temple where you meet them. Zelda's is the Temple of Time.
 * Sheik mentions that Link really does look like the Hero of Time when they meet. Though Navi's dialogue, and the Ocarina of Time manga, both allude to the Master Sword's legacy preceding Ocarina of Time; Zelda shouldn't know what he once looked like, unless she is connected to her memories of a life before.
 * Even without the Triforces, both Zelda and Link have foresight in their dreams. Zelda's dream however differs vastly though, in that her's brings a symbol of hope, as a light that changes into a figure. A witting player might infer this isn't a reference to current meeting, but actually to the ending of Ocarina of Time, where Link returns from the future darkened by black clouds to meet Zelda again with only the Spiritual Stone of Forest. If this is so, this implies Zelda's omnipresence to see the inevitable future before them (though she did not know).

References from Majora's Mask and after
Hylia is referred to a number of times in Majora's Mask. In The Wind Waker, neither Hylia nor a Goddess of Time are ever brought up, but Zelda's alias "Tetra," is a double entendre referring to both the species of fish, and the Greek number for four. Whether the Oocca of Twilight Princess' avian qualities and nearness to the gods is also a symbolic allusion to her is yet to be confirmed, but likely.
 * She is depicted on the design of the of the clocks as a winged caricature holding unto the earth on one end, opposite the moon.
 * She is mentioned by Zelda in a flashback,
 * She is mentioned by Tatl as she panics at the end of the three-day intro.
 * She is mentioned by a Sheikah man muttering a prayer to her in the Great Bay seasise shelter.

Trivia

 * 1) In the Japanese version of Zelda II, the Trophy of Ruto Town is known as the "Goddess Statue" and appears with arms outspread. The Statue of the Goddess in Skyward Sword has the same pose as the international version.