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Arima was shown to have a very calm personality, he remained unperturbed and composed even when surrounded by several ghouls or facing the monstrous One-Eyed Owl. He was also an overly strict mentor, as seen when he reprimanded Hirako for being off on his attack by one decisecond. As a teenager, Arima exerted the same personality, and also noted his main hobby to be ghoul extermination. He had a very cold and calculating approach to exterminating ghouls as shown when he was about to finish off Renji Yomo, and Yoshimura appeared suddenly; instead of panicking or attacking he merely greeted the ghoul. Arima was shown to respect the dead as he reprimanded Itsuki Marude when he talked ill about Mado after his death.
Despite his stoic demeanor, he was capable of forming friendships, he often borrowed books from Haise Sasaki, and Akira Mado described his feelings towards Sasaki as "parental love." Arima was also the type of person who complimented his ghoul opponents, an example being when he did so when Kaneki damaged his quinque, IXA. Surprisingly while dying he reveals he despised himself for killing ghouls. He admits how happy he is to have done something good by imparting his mission to depose V and become a symbol of hope for ghouls.
Arima was hailed as a genius within the CCG, stemming from his affiliation with the organization since his teens. He was personally selected by CCG's chairman Tsuneyoshi Washuu to be promoted to a 3rd class investigator. Taking advantage of his youth, he took on assignments that regular investigators could not handle. One such assignment saw him working undercover as a transfer student in the 13th Ward, where he became the classmate of Taishi Fura. Sent to eliminate the ghoul Lantern, he rescued Taishi and the two formed a partnership to hunt the ghoul down. Their activities soon caught the attention of several ghouls in the ward, as two high school students killing ghouls was unheard of before.
In the passing years, he partnered with Take Hirako.[13] The pair took part in the extermination of the Clowns in the 4th ward, coming into conflict with Uta's gang. There, he was confronted by the younger brother of Hikari Kirishima, Renji Yomo. Before he could deliver the finishing blow, Yoshimura interrupted and rescued the young ghoul from him.[14]
After being given the mission to ambush any fleeing ghouls and neutralize the One-Eyed Owl, Arima was stationed within the underground Route V14, where he encountered and single-handedly killed dozens of ghouls. Upon having his gaze fall on Ken Kaneki, he quickly approached him with the intention of completing his mission and, without warning, dealt an attack towards Kaneki that tore a large chunk out of his abdomen. Before Kaneki could recover and fight, Arima lunged his quinque IXA through the back of Kaneki's skull, which pierced his brain and destroyed his left eye.[17] He easily dodged Kaneki's rapid kagune strikes and witnessed a sudden emotional outburst from Kaneki, during which Kaneki recited a Hakushū poem. Apparently moved by the poem, Arima made note of Kaneki's identity, then offhandedly mentioned that he could tell it was raining outside from the sound of water pouring in from above. He stated that his sense of time got dulled when he was underground.
Arima then brought out his other quinque, Narukami, and activated it. He declared that his objective was to ensure no ghouls would pass through Route V14, and released bolts of Rc projectiles towards Kaneki. After Kaneki dodged those attacks, Arima commanded his quinque to go into remote activation and continued to strike at Kaneki, keeping the manic ghoul at bay. He blocked a desperate attack from Kaneki's kagune using IXA's defensive barrier, but was surprised that Kaneki's blow managed to break the barrier, and so complimented his opponent. Arima went on to impale Kaneki through the abdomen with IXA, lifted him high into the air briefly before he threw him back to the ground. Arima stated