Nix

Nix is the protagonist of the stories Kingdom Hearts: The Final Heart and Kingdom Hearts: The Final Rebirth (this story has yet to be released) as the adoptive sister of the three Keyblade bearers Terra, Aqua and Ventus from Kingdom Hearts: Birth by Sleep.

Kingdom Hearts: The Final Heart
Nix is a small girl with long brown hair and magenta/purple eyes. She dresses in a plain white dress when she lives on Destiny Islands and continues to travel around in the dress until she is captured by Oswald the Lucky Rabbit who makes her wear a white leotard with small black markings all around it.

When she is possessed by Mana crystals of different size appear around her eyes and cheeks as well as her back while her hair turn from brown to a pinkish white. When Oswald captures her once again she is forced to wear a bride-like version of the leotard she wore earlier in the story.

Kingdom Hearts: The Final Rebirth
In the sequel Nix now wears a dark tanktop that stops a few inches below her chest along with beige shorts and a beige form of scarf around her neck

Personality
Nix is somewhat hesitant when she first appears and is constantlt plagued by the guilt of her lack of abilities to go and find her lost "siblings" which has lead to her being the oddball and scapegoat of the village much to her guardians' dismay. After meeting up with Mickey, Sora, Riku, Donald and Goofy she starts getting more confidence and starts trusting people enough to talk about her problems.

Physical
Keyblade Wielder: Although the name of her Keyblade is unknown Nix is able to wield it - also while Mana has control over her body - with great skill and has shown that she is able to defeat some of the strongest Heartless.

Magic: Nix starts out being very poor at magic and several time she blow things up instead of doing the purpose she intended it to do. As the story progresses she makes improvements although not all are as good.

Mental
Possession: This is more of an effect of having another Soul resting inside her Heart but as time goes by Nix is able to contact Mana and draw on her strength when her own fails. This however makes Mana fill more and more of Nix's heart thus erasing her from existence.

Oxus
Nix and Oxus were promised to each other when they both were infants and since Oxus got to know her better he fell deeply in love with her. All of these emotions are returned by Nix who is willing to protect him from anything - even if it means sacrificing her own life.

Terra
Nix sees Terra as her beloved older brother and will do anything to find a way to save him from Eternal Darkness. In some ways he is her father-figure.

Aqua
As Nix's mother-figure Aqua likes to smudder her adoptive sibling in love and both Nix and Aqua tends to exaggerate this to the extreme sometimes.

Ventus
Nix was always protective against her youngest "brother" despite him being older than her and she continues to be like this throughout the entire story.