Itsuki Yakumaru, nicknamed "Ya-kun" by her comrades, operates as a member of Scavenger within Academy City. This group executes ruthless missions involving elimination and destruction. Itsuki's distinctive attire features a loose square-neck blouse, black spats, and a large brown newsboy cap used to conceal her long brown hair and attractive appearance. She carries essential chemical vials on a utility belt.

Personality-wise, Itsuki presents a more serious front than teammate Naru, yet shares the group's underlying childish nature. She approaches missions with detached professionalism, eliminating targets or obstacles to Scavenger's objectives without hesitation. She harbors a specific hatred for teachers, viewing them as the source of evil in Academy City. Her perceptiveness, sharpened through long-term teamwork with Scavenger's leader, allows her to anticipate the leader's strategies, such as deducing plans after observing interactions with adversaries like Misaka Mikoto.

Itsuki wields the ability "Mix Master," enabling her to manipulate chemical reactions within specialized spheres. She creates corrosive substances to melt metal, generates coolant to immobilize targets, and produces explosive or thermite reactions for combat. She applies these abilities tactically during coordinated attacks, such as Scavenger's assault on DA headquarters and their confrontation with Accelerator, where she prepared liquid thermite for a combination strike.

Beyond combat, Itsuki possesses skilled acting capabilities. She convincingly manipulates others by feigning vulnerability—dirtying her clothes and simulating injuries. She notably employed this deception during the evacuation of Skill-Out members, falsely warning of a poison gas leak to efficiently clear an area.

Throughout the Necromancer Arc, Itsuki participates in Scavenger's raid on DA facilities, emphasizing the need to complete missions for payment. She eliminates a surviving DA member (a Tarantula mobile weapon driver) using coolant, though teammates later criticize the kill as incomplete. During the battle against Accelerator, her thermite attack fails, leading to her defeat when Accelerator flicks her forehead as she attempts to reposition.

In the Dream Ranker Arc, Itsuki assists in tracking a robotic doppelganger of Kuriba Ryouko. After a confrontation at Shinasame University's DNA lab, she improvises traps and explosives to delay the doppelganger, prioritizing teammate safety during retreats. She supports the leader's manipulation of Misaka Mikoto into a temporary alliance, gagging Naru to prevent accidental revelation of their deception. Later, she orchestrates the Skill-Out evacuation through her acting and participates in coercing a client using the doppelganger threat. The arc concludes with her discussing team improvements and celebrating Scavenger's reinstatement to A-rank status; she advocates for replacing outdated equipment while retaining sentimental items like Gekota communicators.

During the Jailbreaker Arc, under the leader's orders, Itsuki covertly supports Mikoto's team in the Second Reformatory's escape trial. She sabotages security robots using rust-inducing chemicals, observes interactions between participants like Kuroko and Saten, and grows suspicious of infiltrators like Oumi Shuri. After abandoning Mikoto's group during a robot ambush, she encounters more security units only to discover Mikoto had reprogrammed them. She witnesses Uiharu Kazari's solo success in extracting the fake prisoner, ending the trial.