Mitsuki Konishi commands authority as a high-ranking Reapers' officer and the Week 3 Game Master. Outwardly loyal to Conductor Megumi Kitaniji, her true drive is personal ambition, compelling her to manipulate or eliminate obstacles within the Reaper hierarchy. Her cold, calculating nature and strategic intellect earned her the moniker "Iron Maiden," though Player Beat mockingly dubs her "Ironface." She exhibits zero tolerance for perceived incompetence, deriding those like Beat as "chimps." This haughty perfectionism masks a capacity for explosive rage, particularly when her schemes unravel. While typically detached and cruel, she readily employs violence, erasing Reapers such as 777 and targeting interfering Players.

Konishi presents a distinctive visage: long, wavy blonde hair, blue eyes, glasses, and long, pointed black fingernails. She favors a black maid uniform paired with thigh-high black leggings and white high heels. Accessories include multiple bracelets on her left wrist, a silver ring on her right index finger, and a black claw hair clip securing the back of her head. A subtle black wraparound armband with white frills adorns her upper right arm in specific depictions. Though appearing youthful, her Reaper officer status implies significant age due to the lack of aging.

Her pivotal Week 3 role reveals exploitative tactics. As Game Master, she initially forces Neku Sakuraba into a disadvantage by taking every other Player as his entry fee. When Beat defects to aid Neku, Konishi adapts, issuing a mission for them to find and erase her while she hides within Beat's shadow—a unique ability among Reapers. To distract them, she transforms Beat's sister, Rhyme, into Noise, sealing her within a pin used as Beat's entry fee. She deploys illusions to lure Neku and Beat into traps, mercilessly erases the aiding Reaper 777, and later delegates temporary Game Master authority to Uzuki Yashiro and Kariya. She provides Uzuki with a fake Rhyme pin and a false promise of promotion. Konishi's ultimate scheme involves betraying Kitaniji to survive Shibuya's impending destruction, aligning with the resurrected Sho Minamimoto solely to exploit the chaos and seize the Conductor role. Revealed within Beat's shadow, she is defeated by Neku and Beat with Rhyme's Noise aid. Spitefully, she taunts Beat in her final moments, falsely claiming Rhyme never cared for him before permanent erasure.

In the non-canonical "Another Day" scenario, Konishi surfaces as Beat's school teacher under the alias "Mrs. K." Her demeanor jarringly contrasts the main narrative; she expresses disappointment in Beat for loitering, asks about his homework, and challenges him to a Tin Pin match. She stipulates that her victory requires him to reconcile with Rhyme, displaying apparent concern for their relationship—an ironic reversal of her primary efforts to destroy it. Her Tin Pin deck here features pins named "Tigris," "Popguin," and "One Stroke, Vast Wealth," each with distinct gameplay attributes.

Konishi harbors mutual antagonism with Beat, viewing him as an "obnoxious simpleton" after his early Week 3 interference. Her relationship with Kitaniji is pure deception; outwardly demanding proper deference towards him from subordinates, she secretly plots his downfall to usurp power. Her abilities encompass transformation into her winged tiger-woman Noise form, Tigris Cantus. Battling this form involves four distinct phases: luring specific Noise types to inflict damage; alternating attacks between her real body and clones while navigating positional swaps; recovering stolen pins by erasing summoned clones; and finally, tracking her concealed form via shadow direction to land decisive blows. Her human-form powers extend beyond shadow concealment, including illusion creation and object manipulation like the Rhyme pin.