Lucy Tsukioka is a supporting character in Ayakashi Triangle. She is a high school girl of mixed heritage who comes from a wealthy family; her father works in trade and her mother is a model and actress, though both are often absent from home. She has wavy blond hair and blue or green eyes and attends Hokusai High as a first-year student alongside her close friends Suzu Kanade and Yayoi Toba, whom she has known since middle school.

Lucy has a reserved and somewhat shy demeanor on the surface, but she is also lively, cheerful, and deeply invested in her interests. She is passionate about photography, taking more than three hundred photos each day in search of images she considers worthy of posting online. She has a good eye for composition and is very persistent in her hobby. At the same time, she is obsessed with occult and otaku culture, particularly aliens and UFOs, and she rejects the existence of ayakashi, the spirits and monsters that populate her world. Whenever a supernatural incident occurs, she explains it away with elaborate alien conspiracy theories. Her speech is marked by a distinctive verbal tic, ending sentences with the syllable "ssu," and she often refers to other people using the honorific "shi."

In the story, Lucy serves as a source of comic relief and a narrative foil. Because she cannot see ayakashi, her misattribution of events to extraterrestrial activity adds a layer of humor and misdirection to the plot. She is usually found together with Yayoi, and the two of them form a trio with Suzu. Lucy develops a close friendship with Matsuri Kazamaki, the series' protagonist, after he saves her during a dangerous encounter in a spiritual realm. This experience leads her to harbor a secret romantic interest in the male version of Matsuri. She also encounters Shirogane, the powerful cat ayakashi, and mistakenly believes he is an alien, giving him the nickname "Shiromatsu-sama" and developing an almost worshipful attitude toward him.

Lucy undergoes gradual development as the series progresses. While she remains unaware of the true nature of ayakashi for most of the story, she becomes more deeply involved in the adventures of her friends, and her loyalty and caring nature become more apparent. She does not possess any supernatural abilities; her notable skills lie entirely in her photography and her strong, quirky personality. Her role is to provide a normal, skeptical human perspective within a world full of the bizarre, while also being a warm and reliable friend to the main characters.