Being a school guardian is a rather hard and difficult job. It mostly involves protecting the vampires from humans (ironically enough). Mostly because of the girls from the day class who are quite in love with the guys from the night class. They will do anything to get a chance to talk to the night class, even things that are dangerous. So in order to protect the girls from themselves, Yuki has to be quite strict to the girls and even the vampires. Regardless of how the girls feel towards Yuki, or what they tell her, Yuki will yell at them to keep back and to keep in place. At times she can feel quite frustrated and become a bit harsh with the day class, but when Kaname shows up, Yuki becomes her cheerful self once more.

When it comes to being aggressive with other people, Yuki will only do so when she requires something, is trying to show someone she truly cares, and when she's had a blood hallucination. For example, when Yuki
told Shizuka to drink her blood in order to save Zero, she said it demandingly even though Shizuka was a pureblood vampire. This was Yuki's way of showing she cared for Zero even if she believed he hated her.