Inga emerged as a supernatural entity from a sealed statue within an underground ritual site or cemetery located in a war-torn region. Awakening occurred when Shinjūrō Yūki's blood contacted the statue during a car accident, resulting in immediate possession of his body. During this initial event, Inga ravenously devoured the souls of multiple accident victims, uncovering their hidden truths in the process. A critical intervention followed when Yuuko Kurata took her own life to prevent Inga from consuming her soul, asserting that human hearts hold multiple truths, not one absolute. Inga subsequently attempted, yet failed, to inhabit Yuuko's lifeless body. This outcome prompted Shinjūrō to propose a binding contract: Inga would cease killing humans, consuming only their "screams" (interpreted as souls or truths) without taking lives. In exchange, Shinjūrō would guide Inga to environments rich in deception and unresolved truths for sustenance. Inga agreed, adopting a new form influenced by a film strip in Yuuko's possession at her death.

Inga shifts between two primary forms. The child form manifests as an androgynous teenager aged 14–15, featuring pale skin, purple eyes, and purplish-grey hair covering one eye. Distinctive jet-black skin markings appear on the left arm, right leg, and right cheek. This form typically wears cargo pants, a pink shirt, a black waistcoat, and a white panda-themed hat with attached mittens, projecting a playful, excitable, and loyal demeanor. The adult form presents as a voluptuous woman resembling Yuuko Kurata, retaining the purple eyes, purplish-grey hair, and facial markings but with the hair uncovered. Attire consists of an asymmetrical black top with one red sleeve, black hot pants with one red leg, and a modified panda hat worn as a fluffy shawl. This form exhibits a more seductive, manipulative, and occasionally violent personality. Transformation occurs voluntarily, often triggered by proximity to concealed truths or the need to employ specific abilities.

Inga wields several supernatural powers: shapeshifting for partial or full bodily alterations, extending limbs to access confined spaces, and rapidly healing fatal injuries (demonstrated on Shinjūrō). Enhanced strength and speed enable actions like shattering reinforced objects or intercepting attacks. The most significant ability is truth extraction: in adult form, Inga asks one question that compels absolute honesty from a target through physical contact, eye contact, and a visual manifestation of purple butterflies entering the subject. Described as "devouring screams," this process works only once per individual and requires precise questioning to reveal meaningful truths, adhering to the contractual limitation against lethal soul consumption.

Lore clarifies the entity's nature: Bettenō, another being from the same sealed site, identifies Inga as a "demon born from the hearts of forsaken warriors" who deserted a distant battle. Bettenō claims responsibility for originally sealing Inga, framing this mythological origin as an embodiment of despair or regret from abandoned soldiers. Post-contract, Inga's actions focus on assisting detective work, frequently displaying protectiveness toward Shinjūrō. Their dynamic includes playful teasing in child form and more adversarial interactions in adult form, underpinned by mutual dependence. Beyond the prequel, Inga maintains the child form as default, utilizing the adult form during investigations for truth extraction. The entity coexists with an R.A.I. (Real Artificial Intelligence) named Kazamori, indicating adaptability in forming non-human partnerships. Panda-themed accessories remain a consistent aesthetic trait across all media.