A student of the Night Class, she is Kaname's unofficial bodyguard. She is usually the first to defend Kaname from anyone she regards as a threat. For example, she barely refrains from killing Zero when he points a gun at Kaname for comparing Zero to a level-E. It seems she also acts as a spy for Kaname.   from http://vampireknight.wikia.com/wiki/Seiren

Cross Academy is an elite boarding school with two separate, isolated classes: the Day Class and the Night Class. On the surface, Yuuki Cross and Zero Kiryuu are prefects of the academy, and attempt to keep order between the students as classes rotate in the evenings. As the night class is full of utterly gorgeous elites, this can sometimes prove to be a bit difficult. It is completely necessary, however, as those "elites" are actually vampires. Yuuki and Zero act as guardians, protecting the secrets of the Night Class and the safety of the ignorant day class. 
As the adopted daughter of the academy's chairman, Yuuki takes her job with a serious and energetic attitude. It also allows her to interact with her secret crush and savior, the night class' dorm president, Kaname Kuran. Zero, on the other hand, has a deep-rooted hatred against vampires, and at times, does not hesitate to kill. 
Can vampires and humans co-exist, even in the strict setup of the Cross Academy? Only time will tell. 
(Source: MAL Rewrite)

As a direct continuation of the first season, Zero comes back after his disappearance from the academy. Despite her relief, Yuuki, whose past is still shrouded in mystery, wonders what exactly happened to Zero during the separation. As Zero continues having the strange vision that appeared since the day he drank Kaname&rsquo;s blood, he visits the now awakened Maria Kurenai in an attempt to find answers. She only provides him with a cryptic answer that suggests who the true enemy really might be. 
In the meantime, vampires from the Supreme Vampire Council arrive at the academy in order to punish Zero with death for his sin of killing a pureblood. However, Kaname intervenes with the council's decision with not only the intention of saving Zero but with another purpose unbeknown to everyone around him. 
[Written by MAL Rewrite]

Cross Academy is attended by two groups of students, the Day Class and the Night Class. At twilight, when the students of the Day Class return to their dorm, they cross paths with the Night Class on their way to school. Yuki Cross and Zero Kiryu are the Guardians of the school, protecting the Day Class from the academy’s dark secret—the Night Class is full of vampires!
 
(Source: Viz Media)
 
Note: The chapter count includes 15 extra chapters.