When Sarah Maas arrives at Camp Cold Lake, she is treated poorly by the other female campers. In order to get sympathy from the others, she swims underwater and pretends to drown. When nobody tries to save her, Sarah resurfaces. She finds herself in a ghost world. Sarah hears someone singing Camp Cold Lake's official song. Sarah follows the voice and finds Della. Della greets Sarah by name and asks Sarah to be her "buddy"; according to the camp's rules, every camper at Camp Cold Lake is required to have a buddy. Sarah is only able to exit the ghost world when she is resuscitated by a counselor. Eventually, Sarah's bunk-mates apologize for being mean to her. However, Della continues to appear around Camp Cold Lake, but she is seemingly only visible to Sarah.
Sarah is convinced that Della is the ghost of a drowned camper. This theory explains why the camp has implemented such strict rules. Della begins appearing repeatedly. She demands that Sarah be her buddy. Sarah assumes that Della will try to drown her to make her a ghost as well.
After Della attempt to hit Sarah with a motorboat, Sarah runs away into the woods.
While she is trying to find her way through the woods, Sarah is met by Della. Della begins explaining how she actually died to Sarah.
Della reveals that she had tried to scare Sarah away from the lake so she would go into the woods and die the same way she did. A snake begins coiling around Sarah's leg. Briana, another camper, appears and removes the creature. Briana explains that Della had tried to kill her during the previous summer, and now she is back to protect other campers from Della. Sarah and Briana agree to be buddies. Della wails and falls to the ground before disappearing completely. Briana says that she is unsure if Della is gone forever.
One night, Della sneaked into the woods by herself because she was tired of listening to the water safety lecture. Della did not know that many deadly poisonous snakes lived in the woods. She died by a snakebite.
When Della was alive, she was slightly mischievous, which is made evident when she sneaks out alone at night to explore the woods.
In her afterlife, Della is malicious. She tries to kill campers so that she can have a buddy.
Della has long curls of white-blond hair flowing down the sides of her face and pale blue eyes.