Ugallu emerged from a failed guardian spirit summoning ritual performed by Mikan Hinatsuki's father a decade prior. Ritual errors—an incorrect magic circle, a low-quality vessel, and citrus-infused offerings—caused Ugallu to inhabit Mikan's body instead of manifesting independently. Confined within Mikan and blind to the outside world, Ugallu misinterpreted her purpose. She lashed out with destructive magic against anything causing Mikan emotional distress, creating the phenomenon called Mikan's "curse." This curse posed a severe threat until Sakura Chiyoda intervened, casting a barrier that transformed its effects from harmful into comedic misfortunes.

Ugallu manifests as a young girl with a beast-like appearance, standing approximately 140 cm tall. She has messy brown hair with black spots, long bangs covering her right eye, and a round ahoge. Brown fur covers her limbs up to the elbows and shins, featuring paw pads on her hands and feet. In her battle form, the fur extends further and she wears a short brown sleeveless dress with a tufted collar. After her proper summoning, this form became her permanent physical vessel.

Her personality is energetic, simple-minded, and childlike, often displaying animalistic tendencies. She speaks in broken sentences and refers to herself as "ore." Highly loyal and eager to please, she thrives on praise for completing tasks but despises failure. Despite her simplicity, she exhibits perceptiveness and a strong protective instinct toward Mikan. Cognitive limitations include an inability to read proficiently and difficulty counting beyond ten, though she memorizes vocabulary related to citrus due to Mikan's influence. She adores meat but recoils from citrus fruits.

The curse's escalation during Sakuragaoka High School's sports festival preparations prompted Mikan's isolation. Yuko Yoshida and Momo Chiyoda intervened, entering Mikan's subconscious to confront Ugallu. Initially a hostile miasma, Ugallu solidified into her beast-girl form after Yuko transformed her weapon into a giant whisk. Dialogue facilitated by Momo's snacks led Ugallu to realize her attacks harmed Mikan. Overwhelmed by guilt, she prepared to vanish until Mikan stopped her, urging atonement. The group then orchestrated a corrected summoning ritual with community assistance: Shirosawa provided fur for a high-quality vessel, Lico prepared mana-infused offerings, and friends drew an accurate magic circle. This ritual successfully anchored Ugallu in the physical world.

Following her summoning, Ugallu moved into Mikan's apartment at Banda Terrace. She undertook household chores like cleaning and guarding but sought formal employment to fulfill her role as a familiar. Yuko helped secure a trial at Café Asura, though her claws damaged equipment and illiteracy hindered menu reading. Shirosawa advised focusing on her strengths. During this period, she devoured a mana-corrupted refrigerator to resolve a crisis. She initially regarded Yuko as her "boss" due to her rescue role but later transferred this title to Shirosawa, calling him "Boss of Boss."

Ugallu wields significant magical power derived from Mikan. Her claws slice through solid objects like kitchen counters, and she possesses formidable combat capabilities against other spiritual entities. As a conceptual being without a soul, she can enter dreams, sealed spaces, or subconscious realms as an "offering"—a trait shared only with entities like Yuko and Lilith.

She views Mikan as a mother figure, addressing her with familial affection. Her loyalty extends to Yuko and Shirosawa as respected authority figures. Integration into Sakuragaoka's community is marked by earnest efforts to contribute, assisting at Café Asura or participating in group activities.

Ugallu's name references the Mesopotamian ugallu, a mythical household guardian. Her abilities and protective role align with this inspiration, though she is not the literal entity. Quirks include bringing back croquettes during errands regardless of requests and rejecting meat offerings with citrus.