Yuki was the mysterious little ranch girl who lives on a remote island on a ranch where the giants celebrated. She was responsible for starting a duel between Dorry and Brogy.
She appeared in a small flashback at the end of the Little Garden Arc over 100 years ago in 1422 AOS. When Dorry and Brogy caught two huge Sea Kings, she bet them to see which is bigger.
This bet became a duel and eventually the reason for their duel was forgotten. It is unknown what happened to Yuki after that but since this event was 102 years ago, it can be presumed that she is now deceased.

First Appearance:Chapter 129; Episode 78

Source: