Zeta (ゼータ, Zēta; Greek Ζήτα), born Lilim (リリム, Ririmu), is the seventh person to join Shadow Garden and the holder of the sixth seat among the Seven Shadows, the elite founding cadre of Cid Kagenou's secret organization. She serves as the head of Shadow Garden's reconnaissance, exploration, and espionage division, the unit responsible for covert operations, intelligence gathering, and overseas missions. Her codename follows the organization's convention of naming members after Greek letters in joining order. She is a therianthrope (beastkin) of the Golden Leopard Clan — sometimes rendered as the Golden Panther clan — and a direct descendant of the legendary "Beastkin Hero" Lily (Lili), whom she strikingly resembles, much as Alpha resembles her own ancestor Olivier. She is seventeen in the current timeline (around fifteen at her introduction), her birthday is February 22, and in the anime she is voiced by Ayaka Asai in Japanese and Dominique Meyer in English.

In appearance, Zeta is consistently described as beautiful, with long blonde hair (sometimes depicted as pale white-blonde), large feline cat ears, purple eyes, pale skin, and a long golden tail that, along with her ears, draws particular admiration. Her standard outfit is the Shadow Garden slime bodysuit — a black suit made of slime with roughly 99% magical conductivity — worn with a large cape draped over her left shoulder and a stylized jacket that wraps her arms and upper torso, opening below the sternum to expose a diamond-shaped section of skin at the chest, in the manner of her fellow Seven Shadows. Her jacket is short in front, ending around the ribs, but trails down to calf length in back, accented with puffy fur at the collar and waist. Her leggings are asymmetrical, with gold rings on one thigh and parallel belts on the other. When traveling the world on missions she dresses for the setting — khaki explorer's gear in a jungle, heavy winter clothing in a snowy tundra, and on at least one occasion a cute sailor-style outfit by personal preference rather than necessity.

Where most therianthropes are stereotyped as wild, untamed brutes, Zeta defies that image: she is intelligent, calm, level-headed, calculating, and strategic. She is by nature a loner who strongly prefers to carry out her missions alone, cooperating with teammates only when a situation genuinely demands it or when ordered to. She is quiet, reserved, and self-contained, with a sharp mind well suited to deep-cover work — yet beneath that composure runs an intense emotional core that surfaces around three subjects in particular: demons and the Cult of Diablos, her lost clan, and Shadow himself. She is often noted as one of the most naturally gifted members of the entire organization, a "pure genius" or "Heaven's Gift" capable of picking up almost any tool, weapon, or technique flawlessly on first contact. Cid himself describes her as talented at whatever she does. Her fatal flaw, however, is fickleness: she grows bored the moment she understands something and discards it before achieving true mastery, so her skills, brilliant as they are, often lack a final layer of refinement. It is said that if she didn't habitually throw away her swords right after using them, her swordsmanship would rival Alpha's.

Her backstory is among the darkest in Shadow Garden. Her entire Golden Leopard clan was exterminated by the demon Petos of the Cult of Diablos — her father, her mother, and her infant younger brother all killed — and Zeta herself came perilously close to being enslaved by the cult. This trauma left her with a hatred of the Cult of Diablos far more personal and absolute than that of her comrades; she would sacrifice anyone and anything to see it destroyed, with the sole exception of Shadow. Cid saved her from this hopeless situation, gave her strength, and handed her a new name and a new purpose, and like the rest of the Seven Shadows she repaid him with undying loyalty and reverence, addressing him as "Master." In her case that devotion crosses into outright fanaticism. She is self-sacrificing to a fault — among her defining lines are "I'll shoulder all sin," "If the time comes, cast me aside," and "I'd let the whole world burn, if that's what it took" — and she sincerely intends to bear every consequence and every moral stain on Shadow's behalf.

This is the root of her most dangerous trait. Believing that Shadow Garden under Alpha's leadership is too restrained and too soft in its methods, Zeta secretly founded her own dissenter sub-faction. Her goal is to resurrect the demon Diablos ahead of the cult's own schedule and use it to grant Shadow immortality — to elevate him into a godlike being who would rule and police the entire world forever, ensuring no organization like the Cult of Diablos could ever rise again. She is notably the only one of the Seven Shadows aware that Cid (as Shadow) has spoken approvingly, even philosophically, of eternal life, and she takes this as license to pursue the plan on his behalf. She reasons that Shadow's own kindness is the one thing holding him back from his ambitions, so it falls to her to circumvent that limit by taking matters into her own hands — even at the cost of her own life, which she regards as her purpose. (The series plays on the irony that Cid would actually reject the plan, not from kindness but from sheer pride: he is far too proud to rely on borrowed power and would rather achieve immortality by his own methods.) She softens noticeably in Cid's presence, becoming warmer and more affectionate than her usual cold demeanor, and she holds a quiet reverence toward Claire as Cid's family, feeling real regret over needing to use her in the Diablos scheme but remaining resolute regardless.

Within the group dynamics, Zeta has a running rivalry with Delta — she a cat-type beastkin, Delta a wolf/dog-type — and the two bicker, argue, and brawl constantly whenever they meet, a noisy cat-and-dog chemistry born of Zeta's short patience for Delta's impulsiveness and lower intelligence, though there's an odd animal familiarity underneath it. Otherwise she gets along with her comrades and, in supplementary material, is credited with encouraging Beta to pursue writing novels. Because of her overseas missions and the demands of running her own division, she spends far more time away from Master than the others and has comparatively little screen time, operating mostly in the background — yet her few displayed feats mark her as one of the most powerful Shadows, frequently argued as second only to Alpha.

Her signature ability is Mistform: she can turn her body into mist to hide, reposition, and slip past lethal attacks, and she uses the same power offensively, invading an enemy's body as gas and then rapidly expanding to cause catastrophic internal damage or death. She can also alter the mist's state, coagulating it into numerous floating blades. Her control isn't perfect — the defensive form sometimes prematurely materializes body parts. Beyond Mistform she fights with slime swords summoned from her bodysuit (capable of stretching and extending from any part of the suit), follows Cid's analytical swordsmanship style but prefers to end fights quickly, and as a therianthrope possesses enhanced senses and perception that outstrip even a magically-augmented human's.