Excerpt from the Code Lyoko Wiki: 

A fine observer, she is adept at understanding people and emotion. Good at sports, very smart (as seen by her responses to difficult situations), a hundred percent natural, she is the perfect opposite of Sissi and is very popular, although she seems oblivious to it. Also differing from girls her age, she rather prefers the company of boys, with the only girl she is shown to be close with is Aelita. Yumi is also a very applied pupil, who excels particularly in the literary subjects. Considered as having a great scholastic potential, she has the respect and trust of her professors which can be seen throughout the series.