Inside the labyrinth walls There lies a tiny child who sleeps alone And as the daylight falls The wind becomes so wild across the stone
-My Medea (Vienna Teng)
Not much is given in way of canon history for ICO/Shadow of the Colossus. Both stories are obviously linked, SotC being the beginning of the 'Horned Child' curse and ICO being the end of it after an unspecified period of time. Most everything else is speculation.
Yorda, it can be assumed, was created by her mother, a dark sorceress that lives in a secluded fortress where children born with horns are taken to be sacrificed. The sorceress, reaching the end of her life, plans to use Yorda as an empty vessel to take over when she dies. Yorda was never supposed to wake up from her sleep in the cage she was kept in or develop a will of her own. However, that's exactly what happened when a boy who was to be sacrificed stumbled upon Yorda's cage and set her free.
The end of ICO is ambiguous at best. Some speculate that both children met their demise after the death of the sorceress and are meeting again in the afterlife. Others believe Ico is merely dreaming of meeting Yorda again. I prefer to remain optimistic in my belief that Yorda made a conscious decision to survive and made her way back to Ico.
Yorda's Personality Yorda, as shown in the game, can be both exceptionally perceptive and very naive. Yorda, not having any proper existence before, now is eager for knowledge and exploration and soaks up anything she's told like some sort of glowy white sponge. She does have some knowledge of her mother's powers, and things pertaining to the curse and the castle, but while she knows these things, it's unclear if she entirely understands them.
Yorda's Powers Yorda's power I could never imagine being much use offensively. She is the light to her mother's shadow so I imagine where her mother's powers were mainly offensive, Yorda's would be more passive. Much of the game dealt with Ico and Yorda making paths for each other. Yorda's power to move the pillars that blocked their exit and opening the huge gate doors was integral to their escape. For the most part I believe Yorda's ability lies in opening and divining things, her powers growing to suit her curious nature.
Yorda's speaking Yorda has picked up the basic local language frighteningly quickly. Still, lack of grammatical understanding, limited understanding of certain concepts and the fact that english is a messed up language anyway, her manner of speech is disjointed and simplified.
Garcian Smith: The first person Yorda talked to. She finds him very kind and already looks up to him with something akin to hero-worship.
Zexion: He was the first person Yorda met upon arriving in Discedo and since he has been the most helpful in providing means in which she can learn, Yorda trusts him implicitly.
Demyx: Yorda is awed by Demyx's musical abilities. Like Zexion, Yorda's learned a great deal from Demyx and she's a little star-struck by him.
Peter Pettigrew: Yorda finds Peter intriguing and fun to talk to. She wants to find out more about this odd person.
Sweeney Todd: Like Peter, the barber fascinates Yorda, though she is somewhat intimidated by him.
Linda: Same as Demyx, Yorda is awed by Linda's talent, and likes her cheerful and outgoing nature. She's very curious about Linda's horns as well.
Umi: Yorda sees Umi much like a role model. Umi's skill in fencing and self-sufficiency are a source of inspiration for Yorda as she struggles not to be a burden.
Mona Lisa: Yorda has come to associate Mona Lisa with trees and earth, and she finds the older girl's strength and kindness very reassuring.
Prydain (Replica Riku): Yorda relates very much to Prydain, the key to his existence very much an aid to seeking answers to her own questions. She counts him among her dearest friends.