Yahiro Takigawa serves as the human alias for Lars, a demon conducting covert operations. Initially posing as an amicable student at Hijirigasaka Academy, he befriends Basara Toujou despite male peers' resentment over his closeness to Mio Naruse and Yuki Nonaka. In this human guise, he maintains an unremarkable appearance with black hair and purple eyes, wearing standard uniforms or casual outfits during social engagements.

His true persona is Lars, a high-ranking demon double agent infiltrating the Demon Lord Faction for the Moderate Faction. As Lars, he dons a formal black suit and a white mask adorned with gold designs, revealing demonic traits like slit eyes and elongated ears. His core mission revolves around observing Mio Naruse under his student cover.

Lars originated from a demon-realm orphanage managed by Mio’s foster parents, who nurtured war orphans. He held deep admiration for them and fellow orphans. After uncovering that the demon Zolgear murdered Mio’s guardians to awaken her Demon Lord powers, Lars committed to vengeance. This pursuit defined his actions, leading to Zolgear’s prolonged torture and death—a deed concealed from faction leadership.

His personality fractures between identities: Yahiro displays laziness, employing magic for schoolwork while exuding friendliness; Lars embodies ruthlessness, cold calculation, and manipulative strategy as the "Masked Demon," attacking Basara, Mio, and Yuki to preserve his cover. Yet his aggression often masks protection, deliberately inflicting non-lethal wounds. He channels fierce dedication toward ending war orphans' suffering, diverting faction funds to orphanages, and frequently mocks Basara’s harem dynamics.

Lars wields formidable demonic powers, ranking among both factions' elite. His signature ability manifests iridescent Magic Spheres of extradimensional matter—manipulated for high-speed assaults, explosions, and barriers that pierce certain defenses, as proven against Kouryuu. He matches Basara’s speed, endures slashes from the cursed Brynhildr sword, and masters summoning, magic circles, and Shadow Clones. These clones serve offense, defense, and espionage, even replicating others’ powers. His acting prowess deceives high-ranking demons like Leohart.

Character evolution exposes deeper loyalties: his vengeance springs from genuine grief over his adoptive family’s demise. Later volumes reveal his willingness to expend life force for allies, notably aiding Basara’s group. The OVA *Departures* depicts an anime-original spar with Basara, testing the latter’s resolve before a village return, underscoring their complex alliance beyond factions.

Beyond missions, Lars retains ties to an unnamed adoptive brother and sister from the orphanage, plus allies like Noel. His relationship with Basara shifts from hostility to mutual respect, often sharing barbecue meals. He interacts pragmatically with others, tolerating some while likening figures like Natsuki to Maria’s succubus persona in annoyance.