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. She is highly skilled at various martial arts as well as ninja-like weapons. Seiren can seem cold, but is very loyal. She is a very quiet and enigmatic character who only speaks when necessary.

Serien was originally named and born as a human girl called Hoshino. She was bought as a infant and raised in the way that, she would become a sacrifice for Kaname to feed on. When offered up to Kaname, she and another sacrifice, Yoruno, Hoshino gladly offered herself to him. She believe that by letting Kaname drink her blood, she would be allowed to go to heaven. Kaname however recused both Hoshino and Yoruno by taking them to Cross Academy. However, on their way to Cross Academy, Yoruno runs back, as she fears that if she doesn't offer herself, she will go to hell. Hoshino runs after her in order to stop her, only for the both of them be caught by aristocrat vampires. While Yoruno dies from the loss blood, Hoshino is on the verge of death when found by Kaname. He offers her the option to die or to live the life of a vampire. She choose to live, and Kaname turns her into a full fledged vampire, while giving her a new name, Seiren. To show her gratitude, she trained and stayed by Kaname's side.
(Source: Vampire Knight Wikia)