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Keiji was a loyal soldier of the Survey Corps. He seemed to have an impatient personality, as seen when he hit Annie's crystal enclosure repeatedly while accusing her of paying for her crimes and yelled at Eren for leaving his position when killing a Titan.
Keiji is first seen participating in the operation to capture the Female Titan inside the Titan Forest during the 57th Expedition Outside the Walls. Commander Erwin Smith calls him to prepare the explosives to blow off the Female Titan's hands, which are protecting its nape. Keiji expresses concern that doing so would also blow up whoever is inside, causing Erwin to modify his plan, and order Keiji to set the charges farther back, so that they will only sever the Titan's wrists.
After Annie Leonhart reveals herself to be the Female Titan and transforms inside of Stohess District, Keiji and his squad pursue her, trying to stop her from escaping over Wall Sheena. However, he and his squad are forced to break off their pursuit when she begins destroying buildings, sending debris flying in their direction.
In Ehrmich District, Hange goes over a report about Annie Leonhart's background with a number of soldiers, including Keiji. He hears Armin, Eren, and other members of the 104th discuss whether it is possible that Reiner Braun and Bertolt Hoover could have been working with Annie.
Keiji is among the soldiers that accompany Hange Zoë to rescue the 104th graduates trapped at Utgard Castle. When Eren ignores orders to stay out of sight in favor of killing his first Titan in his human form, Keiji berates him for breaking cover.
After rescuing the recruits at Utgard Castle, Keiji returns to Wall Rose with the rest of Hange's squad. As the heavily injured Ymir is being lifted to the top of Rose, Keiji muses that, had she been a normal human, she would not have survived her experience at Utgard. After Hannes brings word that there is no hole inside of Wall Rose, the soldiers prepare to return to Trost District, but are stopped when Bertolt and Reiner reveal themselves to be the Colossus and Armored Titans.Keiji and the other Scouts scatter to avoid the Colossus Titan's attack, but he notices that one of the soldiers does not make it and is eaten by Bertolt, along with Ymir. Hange orders her soldiers to swarm the Titan, and as they dodge around its fist, Keiji notices that the Colossus Titan is slow, just like in reports. He joins the other soldiers in diving for its nape, but before they can reach it, the Colossal Titan emits a scalding burst of steam that drives all of them back.
Hange leaves three of the four squads present to watch Bertolt and takes the remaining squad, including Keiji, to face the Armored Titan, which is fighting Eren in his Titan form. As Eren gains the upper hand and traps the Armored Titan in a hold that threatens to tear its head off, Keiji hangs off the Wall with the other soldiers and shouts encouragement to Eren. Helpless, the Armored Titan roars in desperation and the Colossus Titan responds to the signal. It plunges from the top of the Wall to land on Reiner and Eren, creating an explosion of steam. Keiji is burned by the scalding winds and is laid out among the unconscious wounded on top of Wall Rose.
Keiji is reunited with Squad Levi, Hange, Goggles and Nifa, listening as they discuss Armin Arlert's plan to seal the hole in Wall Maria using Eren's Titan hardening abilities, and as Hange reveals Nick's murder at the hands of the Military Police First Interior Squad.
Soon after, Keiji can be seen with Squad Levi while they speak with Dimo Reeves and Flegel Reeves, listening to the message from Erwin brought by Nifa. He watches as Levi interrogates Historia and forces her to accept her role as the queen. He also hears Nifa speak about Erwin's plan to have Eren and Historia handed from the Reeves Company to the Military Police and track them to the location of Historia's father, Rod Reiss, the true ruler of the Walls.Later on, while a group of members of the Military Police carry two caskets from a funeral home to a carriage, presumably hiding the kidnapped Eren and Historia, Goggles and Keiji monitor the scene from a concealed location, with Nifa and Levi monitoring from a roof. Shortly after, Kenny Ackerman and his subordinates make their appearance and several of them ambush the soldiers, killing everyone except for Levi. Keiji is shot in the head by one of Kenny's subordinates, and collapses in the street, where his body is found by an alarmed passerby.
Later in the Battle of Shiganshina, as Erwin Smith begins to lament the dwindling hopes of his dream, Keiji stands among the many fallen Survey Corps comrades in Erwin's mind, wanting to know if their deaths had meaning.