Haruna hails from the world of Villiers, trained as a Masou-Shoujo by her teacher, Ariel. A mishap during her mission to Earth lands her in Tokyo instead of Kyoto, leading directly to her meeting Ayumu Aikawa. During a fight against a bear-like Megalo in a graveyard, she accidentally slices Ayumu in half with her magical pink chainsaw, Mystletainn. Her subsequent attempt to erase his memories backfires spectacularly, stripping her of her powers and clothing. With Ayumu's necromantic magic absorbing her abilities and sustaining his zombie form, she moves into his residence to monitor him and plan her recovery. She delegates her Megalo-fighting duties to Ayumu, compelling him to transform into a frilly, pink-costumed Masou-Shoujo.

Her sentient weapon, Mystletainn, cuts monsters, trees, and humans. Her signature "Mystletainn Kick" technique involves a chainsaw strike. After Ayumu accidentally destroys Mystletainn, Haruna reveals unexpected mechanical skill by reconstructing its complex mechanisms overnight, though fatigue caused her to miss a critical screw, leading to another breakdown. Mystletainn later recognizes both Haruna and Ayumu as co-owners, allowing them to wield it simultaneously during pivotal battles like the confrontation with the King of the Night.

Haruna combines self-proclaimed genius with selective memory, remembering only what interests her while forgetting crucial assignments or names. She consistently recalls Ayumu's name, however, hinting at deeper regard. She exhibits childish tantrums, frequently assaulting Ayumu over minor frustrations. An obsession with cooking fried eggs defines her domestic life, often preparing entire meals centered on this dish. She adopts a protective, sisterly role toward Eucliwood Hellscythe, though this occasionally manifests as possessive jealousy.

Romantic feelings for Ayumu develop throughout the series. Early signs include jealousy when he accidentally kisses Tomonori, prompting Haruna to attack him and lament not receiving a kiss. She dreams about kissing him, directly requests one, and later sabotages their date by inviting others. Emotional vulnerability surfaces when she mistakenly believes Ayumu is involved with another woman, causing her to retreat home in tears. In the manga's conclusion, she kisses Ayumu twice, declares herself the "winner," and asserts they are "going out," while the light novel leaves romantic pairings open-ended.

Initially an inactive bystander after losing her powers, Haruna gradually regains combat relevance. She collaborates with Ayumu and Seraphim to erect defensive barriers against massive Megalo threats. Her strategic intellect emerges during high-stakes battles, like analyzing Yoruno's light-based weakness, marking her shift from reliance on others toward renewed self-sufficiency.