She was the first victim to have her heart's egg hatched into a X-Character. Her egg was "hardened" due to her worring about moving to America and having to face changes at which point Nikadou transformed it into a X-Egg.

Spoiler

In the end, she was able to face the changes and moves to America. Yuki's Guardian Character is dressed in a teacher's outfit. She is also very good in English.

(Source: Shugo Chara! Wikia)