Yuki Ishida is a former best friend of Myrtle Edmonds and a current good friend to Lilo Pelekai and Darla Dimple. Before Darla's Moon powers take action in Stitch Has a Glitch, Yuki, Teresa, and Elena used to copy and follow Myrtle around until they realized how mean and boring Myrtle was and became Lilo's friends after the story and well into the other movies and the series. Yuki also has a major role in Cherry and Atticus Meet Wendy Wu, as she is revealed to be descended from a brave warrior who must conquer a dragon who wakes every other generation.
Yuki was born to her parents seven years ago. It was mentioned that they were born and raised in Japan and had moved to Hawaii at some point before Yuki was born. Yuki was originally a friend of Myrtle's until Darla realized how mean and boring they were and she used her Moon Power to reform her, Teresa, and Elena. This allowed Yuki and the others to become Lilo's best friends instead of Myrtle's.
Yuki is more of a tomboy than her friends as she is shown to be very partial to hockey and martial arts. At some point in the future as revealed in Akito and Estelle Go to Sky High, she grows up and gets married and has a son who is Prince Kentaro of Kushin who renamed himself Leon.
She was also said to not have actually died when he was younger, but she had to go back to her region in order to train and fight to keep their family safe and she will someday train Leon for the same fate when he is ready which will be very soon.