About

Kalin's younger sister, who's been very sick ever since a young child (estimated to be due to pollution).
With her mother dying soon after she was born, and her father sent to jail during her early childhood, all family Kaori ever knew was Kalin. She grew to be extremely attached to him and always appreciated all his efforts to sustain both of them, while feeling bad about being a burden due to her young age and sickness. So, despite being a naturally lively child, she did her best to stay quiet and not get in Kalin's way, and keep up a smiling and happy facade.

After the orphanage they staid at had to close down, Kalin left Kaori at Martha's orphanage while he lived on his own (Kaori was 9-10); this was because that way Kaori could get some medical care and a safe place to stay at, while he looked for some other solution and worked to make Satellite a safe place.
This way Kalin met Yusei & co., and formed Team Satisfaction. Kaori met them as well, but while she never got along with Yusei and Jack (who acted mainly cool/reserved and uninterested), she started a great friendship with Crow, and an ok one with Lorelai. Crow grew to be quite protective of her, and Kaori appreciated how fun he was to be around, Crow not treating her any differentely because of her condition.

When Team Satisfaction broke up, Kaori still remained friends with Crow (albeit seeing him much less), and after the final tragedy nobody really had the heart to let her know what happened to her brother. She tried to remain quiet first, thinking Kalin (and Lorelai, since she disappeared too) was just busy, but eventually threw so many tantrums they eventually gave in and only told her about Kalin being in prison, but nothing about his death.
Months later, Lorelai got in contact with her from Neo Domino, offering her to let her stay in an actual ND hospital with proper care. Thinking that this way she could visit her brother more easily, Kaori accepted.

Eventually, Lorelai had to give her the sad truth about Kalin's death; Kaori was devasted. She closed herself in, and if it weren't for the nurses she would have wasted away. Invested in reaching out to her brother Jack, Lorelai didn't spend as much time with Kaori as she would, but thankfully not long after Fortune Cup happened and the gang got reunited again.
Even if the guys tried to keep it from her to not give her false hope, Kaori managed to know about Yusei dueling Kalin and finally got the strength to keep up with her health. Her dreams were always nightmares though, and as the gang noticed how they were almost premonitions as to what was happening with the Dark Signers, the Arcadia Movement took interest in her and kidnapped her.

At the Arcadia Movement, Kaori was studied for any possible ESP abilities, and there she befriended Rem, whom she came to consider as a brotherly figure like with Crow. Thankfully her stay at Arcadia didn't break her too much emotionally, as she kept a naive, childish and optimistic demeanor and was grateful to have met some new friends, but she still longed to get out with Rem and finding their freedom, and her brother.
In the meantime, Crow was the most affected by Kaori's kidnapping, and when the gang finally discovered the Arcadia Movement was behind it, their HQ was destroyed during their battle with Misty and Carly. Kaori was nowhere to be found in the wreckage, only her moon necklace (a gift from Kalin, she -never- takes it off and although she doesn't know it, the moonstone in it is what gives her the ability to have dream predictions and visit the Spirit World when asleep), so she was given for dead.
Crow swore revenge for her and the other kids affected by the Dark Signers', but Kaori wasn't really quite dead...

After the Dark Signers' arc (for more on her role in it, see the Spoiler tab), Kaori's health improved a lot and she followed Kalin to Crash Town. This time, almost losing her brother made her willing to fight rather than fall in depression, and while Yusei forbade her from helping fearing for her health, she still got in quite some trouble by stubbornly trying to help anyways.
She eagerly helps bringing the now Satisfaction Town to life, and befriends Nico and West easily, acting as a sort of big sister figure to them. She still has friendly contacts with everyone, especially with Crow and Lorelai who she sees regularly (they often pick her up and bring her to Neo Domino to hang out). Is of no particular importance during the Yliaster/WRGP/Z-One events, even is sometimes hangs around.

While being sheltered and childish so far (often annoyingly so), Kaori thankfully grows up and matures, and eventually moves to another city after falling in love (this is during the final timeskip); she still visits regularly all of her friends and especially her brother, but she's not dependent on them anymore and as the years go by she even has her own family her being Shun Kurosaki's mother, so Ruri's as well, is definetely my headcanon now lmao. So yay, happy ending~!