She used to be the most popular girl there. When Amy Goldman started at the school, she and Arlene became friends, and Arlene's status helped Amy gaining popularity as well.
When Amy and two other girls began to sneak out of school to meet with a mysterious man posing as a vampire, Arlene became jealous, believing that she should have been chosen instead of Amy, and their friendship began to crumpled. Amy decided to play a prank on Arlene and send her a note, forged to be written by the vampire and asked to meet Arlene. Arlene fell for it and snuck to the meeting place, but only to find it empty. Arlene tried to claim that she had met him, but everyone at school knew she was lying and turned against her. And Arlene went to being popular to be a laughingstock.
Angry at Amy for tricking her, she decided to stay in the infirmary during Amy's next visit to the vampire, so she could tell on her, when she got back. But she went out to check on her, she found her unconscious near the fountain. She put her head down in the water and drown her to get back at her.