Ell is Carmilla's ex, and a victim of the vampire cult's sacrifice.
Ell is seen in the dreams of the targets as a crying girl in a white night dress. She is often standing, yelling at the girls to "stay away from the light, the light is hungry."
At the final battle in Dudley Chapel, when Carmilla comes to save Laura and her friends, Ell is seen in the bright light to be reaching out for Carmilla, possibly begging for her forgiveness and asking her to join her and be together for eternity.
Being one of the vampire cult's targets, Carmilla arrived in Ell's life unexpectedly, but made a significant impact. The two developed a close friendship, which soon turned into romantic feelings for one another. Carmilla took great lengths to hide what she was from Ell, and in doing so planned for the two to escape together before the day where she was meant to turn her over to her mother. However, Carmilla's mother found out about this plan and visited Ell, exposing Carmilla's secret in the most horrific of ways. Ell was disgusted and frightened, and when she gave up Carmilla, she was taken away to be sacrificed to the light.
After she is sacrificed to the light, her conscious is able to reach the other intended targets and tries to warn them of the upcoming danger, but they are often regarded as nightmares. After Laura is saved from being taken, that Ell's message is made clear. During the final battle, Ell appears in the light and she sees Carmilla, she reach out for her. Carmilla cries at the sight of her, and then drives her sword into the heart of the light, thus freeing all those trapped in the light, Ell included.
Her name is likely meant to be a phoneticization of the letter L as a reference to the character of Laura from J. Sheridan LeFanu's novella, as the story of Ell and Carmilla is essentially a romanticized retelling of Laura and Carmilla's story from the original book.
Ell was one of Carmilla's targets, and the two developed romantic feelings for each other. When it was time for Ell to be taken, Carmilla attempted to escape with her. But they were discovered in which, Carmilla's mother told Ell about Carmilla being a vampire but told her in a horrifying way in which Ell would see Carmilla as a monster. Ell betrays Carmilla and is then taken away to be sacrificed. Ell's death is the one death that drives Carmilla to stop her mother.
Ell visits the dreams of Laura to warn her about the sacrificial event held by the vampire cult every 20 years.