Lily is a central character in the anime 18if, first appearing as a mysterious white-haired girl that the protagonist Haruto Tsukishiro encounters in a dream world. In this realm, she serves as his guide and companion, often calling him her older brother. She initially presents herself as a cryptic and somewhat playful figure, but her motivations run deeper than mere companionship.

Her primary role in the story is to keep Haruto within the dream world, as she believes he is the only one capable of defeating Eve, a powerful entity whose awakening threatens to destroy the real world. Lily is revealed to be the aspect of Eve before she was corrupted by consuming the fruit from the Tree of Knowledge, effectively making her the uncorrupted half of that being. Throughout the series, she helps Haruto navigate the dream world and understand the nature of the witches, including their condition known as Sleeping Beauty Syndrome, a coma-like state caused by emotional torment in their real lives.

Lily possesses notable abilities connected to the dream world, including the power to manipulate the environment and guide Haruto between different dreamscapes. She also has deep knowledge of the witches and the rules of that realm. Her relationship with Haruto is the emotional core of her arc; she treats him as a brother and protector, and her growing attachment to him influences her actions. Over the course of the series, she develops from an enigmatic helper into a more vulnerable figure, culminating in the revelation of her true identity as Eve. In the series finale, Lily merges with Eve and leaves for God's Domain alongside Haruto, choosing to ascend to a higher plane of existence rather than return to the real world. Her character embodies the themes of sacrifice, redemption, and the conflict between pure intentions and corrupted fate.