I actually made this one right after the first Kindgom Hearts came out.
She (Kodai) is the only daughter of Ansem the Wise. When Radiant Garden became Hollow Bastion, Kodai was kept as a prisoner by Xehanort's heartless because he thought that she was one of the 7 princesses of heart. When she proved not to be, he decided to get rid of her. Kodai somehow escaped from Hollow Bastion and found herself in Traverse Town. She vowed to return to Hollow Bastion and stop him.
She meets Riku, and realizes that he is being controlled by darkness. They talk, blah blah blah, Malificent orders Riku to take Kodai back to Hollow Bastion because Xehanort's heartless realizes that she could be usefull. So Riku tricks Kodai into thinking that he wants to stop the heartless as well, so they travel back to Hollow Bastion, and Kodai then realizes Riku's true incentive. He is then draped in his darkness attire, and they do battle. Kodai tries to talk to Riku, but he is becoming engulfed in the darkness and cannot think properly. She is finally beaten, but manages to escape to the lower levels of Hollow Bastion (prisoner cells) ((btw, she has twin blades, similar to Rikku's from FFX-2)) Riku then finds her and drags her to the experiment chamber where the princess cells are.
She is placed in the middle of the experiment room in front of the giant heart. Xehanort's heartless comes out and greets her (evily, that is
twisted ) welcoming Kodai back to Hollow Bastion. She is tied down, and this is where we find out that Xehanort performed experiements on a younger Kodai before he sucked Radiant Garden into darkness. After Ansem found out that Xehanort had done this, he banished him from Radiant Garden. Xehanort had tried implanting a heartless into a human, and found a good test subject, which was Kodai (5 years old at the time). She is basically half-heartless, and can use darkness, but it can overwhelm her if she uses it too much (kinda like claymores and their youma half taking over).
Xehanort's heartless now wants to extract her heart for further investigation, and right as he's about to take it, Riku realizes what he's doing, and tries to stop him. But Xehanort's heartless consumes him and his body. Kodai finds the strength to get out of bondage and battles the dark Riku. She does some damage, but cannot defeat him. Then Sora comes in, he battles Riku, blah blah blah, and we last see Riku closing the door to Kingdom Hearts.
During the events of Chain of Memories, Kodai is trying to find Riku, and during this, she finds her father. They have a somewhat bitter reunion, because she thinks that he has been hiding for years, doing nothing, while she's been trying to defeat Xehanort. Riku comes into the seen, but at that time he looks like Xehanort's heartless. She doesn't recognize him, but he conceals his real identity, ashamed of his appearence. They all decide to work together, Riku even talks to Kodai about forgiving her father (she doesn't know that he is Riku, however). She then talks about finding a certain boy that she couldn't save, and Riku reveals himself. Kodai cries and embraces a surprised Riku, who thought that she would've despised his appearance. Riku then tells Kodai and Ansem that he must help Sora, so he goes on, and Kodai continues to do research, helping her father and King Mickey. This continues into Kingdom Hearts 2.
During the events of Kingdom Hearts 3, King Mickey summons Sora, Riku and Kairi to his castle, telling them that they need to band together to stop Xehanort once and for all. Kodai is reintroduced to the whole gang, and they go to Radiant Garden, where Kodai has been researching the events surrounding Xehanort in Ansem's study. She has uncovered the identities of 3 keyblade warriors, the last before Riku and Sora. She states that they battled with Xehanort, and that they could still be alive (Birth by Sleep reference). Also, these 3 could teach Riku, Sora and Kairi how to properly wield the keyblade. She has found out that the first person to find is Aqua, who could still be in darkness, and that she would know the location of Ventus (who is in Castle Oblivion, and after locating Aqua, they go to Ventus). But Terra is the hardest to find. As the gang searches for these legendary keyblade warriors, Kodai is trying to uncover where he is. She finally finds files pointing to a suit of keyblade armor, which is where his spirit still lies. The armor is recovered and brought to Radiant Garden, and Kodai touches the armor, sending her into Terra's mind. He has been in isolation since Xehanort took his body. She asks him about his past, blah blah blah. She returns to Radiant Garden, and after experimenting, she helps Terra take form in his armor.
This is what I have so far
biggrin 
Here is what she looks like in Gaia form. Her pants don't have flowers, though, but this is the shade of green they have. Her glasses change from KH to KH3. This is how she appears in KH-KH2. Her mother had brown hair and green eyes, while Ansem has blonde hair and orange eyes.
200
100
200
