- "In The World everyone is blessed. Even you and me."
- — Zefie —
Zefie has the body of a young girl with saffron-colored hair, reddish-purple eyes and a white dress. She can sometimes be seen holding a teddy bear, or a lycoris flower.
When she first met Shugo, she called him, "mommy", but she was really referring to the Bracelet, which has her mother's "scent" on it. Her mother is Aura. She seems to find amusement in annoying all of Shugo's parties members except for him and Mireille. She calls Shugo, "big brother", which usually drives Rena off the edge, and also calls Rena a "Ganguro", or "Kogal" because of her avatar's design. The moment where she shows the most emotion is when she is reunited with her mother. When Kamui calls Zefie a "bug", she asks Kamui "Are you going to delete me again?", apparently confusong Kamui and Albireo, indicating some connection to Lycoris.
Her special attack is her "Loving Boomerang Hook", which can do anything from re-write character data (like with Komiyan III), to simply cheering a person up (like Rena when they were in the Cobalt Knight's jail cells). She can bend the rules of the system like any other AI in The World. Her data is heavily encrypted, making her more or less immune to deletion.
Zefie is first introduced to Shugo and his party after she ruins Balmung's fireworks event by making it rain. Shugo is drawn to the sound of her singing, and encounters her on a rooftop in Mac Anu. Zefie instantly notices his bracelet which has her mother Aura's scent on it. This causes her to think that he's Aura. Even after the truth is revealed, she remains attached to Shugo, much to the displeasure of Rena.
After a rather gaudy display of her control over the system Mireille realizes that Zefie is a Vagrant AI. Zefie reveals that she's Aura's daughter, and that she's looking for her Mother somewhere in the system. Eager to see Aura again, Shugo offers to help Zefie in her search, and she becomes a member of his party. Their search for Aura draws the attention of Kamui of the Cobalt Knights, a debugger with a great hatred for Vagrant AIs. She manages to capture Shugo's party and place them in special holding cells. But Zefie uses her AI powers to open the doors, allowing them to escape. They run to Hulle Granz Cathedral where they meet up with the .hacker Kazu. He gives them a Helba Key which will allow them to run to Net Slum, a place where Kamui can't follow.
Before they can escape, Kamui reappears with several Knights and begins to fight Shugo. She manages to break through his defenses and prepares to delete Zefie with her Spear of Wotan. However upon activating the delete command her spear shatters as a protective barrier appears around Zefie. Realizing that Zefie is undeletable, Kamui begins to question her role as a debugger. But Zefie comforts her, saying that she was only doing what she thought was best for The World, and that The World loves her for it. She then travels with Shugo to Net Slum.
Upon arriving in Net Slum Zefie is confronted with several bugged AIs. She uses her powers to delete them, putting them to rest. One of the AI's she encounters is a remnant of Lycoris, the AI that Zefie was apparently based on. After some encouragment by Shugo Zefie deletes the remnant, which reforms into a lycoris flower. Zefie takes the flower for herself. Balmung appears soon after and challenges Shugo to a duel, which he wins. As a reward for winning Balmung gives him the field address for Aura's current location as well as Virus Cores he needs to get there. With Shugo and Rena by her side, Zefie travels to the field, and is finally reunited with her Mother.
Shortly before The World's servers were integrated into the The World R:2, Zefie traveled to Mac Anu, where she encountered the private character of the programmer Jun Bansyoya. During their brief conversation she told him that Aura had abandoned her role as the Ultimate AI and integrated back into the system of The World itself. After that she "returned to the winds."
- Zefie does not make an appearance in the Legend of the Twilight anime.