Katie made her first appearance in the first episode of the first season of the anime. Katie is a first-year student (indicated by her brown cape) at the Tristain Academy of Magic and is one of the girls that Guiche dates.
It was seen that she brought a food basket for Guiche while Guiche and Montmorency are dating on the courtyard. She searched for Guiche, seeing Saito, he then pointed the location where Guiche was. Montmorency starts getting jealous after seeing her. Because of knowing that Guiche plays two girls at a time, Montmorency slaps his face and he got embarrassed, then she and Katie got mad and both left.
Guiche blames Saito for the embarrassment, after that Guiche challenges Saito a duel. In the novels, it is mentioned that after the Albion war and after Saito halts 70,000 Albion soldiers, Katie praises Saito for his achievements. She also prepares some cooked food and fruit salad for Saito. Katie and Noble girls like her have an argument with the maids of the Magic Academy to decide who will prepare and serve food to the Boys who were involved in the war.
Zero no Tsukaima Season 1 Episode 2 (Katie's appearance)