|Go for it, Eren! Tear the head off his neck! Pull the traitor outta there!|
|— Keiji encourages Eren during the latter's fight with the Armored Titan|
Keiji was a tall, tan soldier with a large build. He had brown hair, which was beginning to turn gray in the back, which he kept in a buzz cut, brown eyes, and had rather defined cheekbones. During missions, he was shown wearing the standard Scout Regiment uniform.
Keiji also seemed to be rather blunt in his speaking, casually commenting that Ymir is not normal due to her Titan abilities while speaking with Eren, seemingly unconcerned with how Eren, an Intelligent Titan himself, would feel.
Keiji is first seen participating in the operation to capture the Female Titan inside the Forest of Giant Trees during the 57th Exterior Scouting Mission. Commander Erwin Smith calls him to prepare the explosives to blow off the Female Titan's hands, which are protecting its nape. Keiji points out that doing so could also blow up whoever is inside, and Erwin modifies the order to amputate its wrists instead. Keiji returns to the rest of his squad and orders them to prepare the special target restraining weapon again.
As the scouts are preparing, the Titan attracts a group of Titans with a loud scream. Keiji joins his fellow scouts in an attempt to slay all of the Titans to keep them from devouring the Female Titan, but they are eventually overwhelmed and ordered to retreat.
After it is reported that Titans have invaded Wall Rose, Keiji is among the Scout Regiment soldiers that Erwin Smith leads from Stohess District to the front lines. They stop in Ehrmich District where Hange goes over a report about Annie Leonhart's background with a number of Scouts, including Keiji.
Keiji is among the soldiers that accompany Hange Zoë to rescue the 104th cadets trapped at Utgard Castle. During the battle, Keiji berates Eren for ignoring orders to stay out of sight when the latter breaks cover to kill his first Titan in his human form.
After rescuing the recruits at Utgard Castle, Keiji returns to Wall Rose with the rest of Hange's squad. While using his ODM gear to help other soldiers climb the wall, he informs Eren of the injuries Ymir sustained during the battle and muses that she would not have survived her experience at Utgard if she had been a normal human. After Hannes brings word that there is no hole inside of Wall Rose, the soldiers prepare to return to Trost District. However, they are stopped when Bertholdt and Reiner reveal themselves to be the Colossal and Armored Titans.
Keiji and the other Scouts scatter to avoid the Colossal Titan's attack, but he notices that one of the soldiers does not make it and is captured by Bertholdt. To his surprise, the Colossal Titan eats both the soldier and Ymir. Hange orders their soldiers to swarm the Titan. As they dodge around its fist, Keiji notices that the Colossal Titan is slow, just like in the 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 Bertholdt and takes the remaining squad, including Keiji, to face the Armored Titan, which is fighting Eren in his Titan form. Eventually, 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.
The Armored Titan roars in desperation and the Colossal 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.
Fully recovered from his injuries, Keiji reunites with the rest of his squad and Levi's squad. He takes part in Hange's experiments on Eren's Titan form, observing the tests from a distance. When the strain begins to damage Eren's body, Hange ends the experiments and orders the soldiers to disperse. As they leave, Keiji orders Jean to check and make sure there are no witnesses.
After Hange discovers that Pastor Nick has been murdered, Keiji accompanies them to inform Levi and his squad. Shortly after bringing the news, Nifa arrives with a message of her own from Erwin revealing that a freeze has been declared on all Scout Regiment activity and Eren and Historia Reiss are to be handed over to the government. Hange places Keiji, Nifa, and Abel under Levi's command so that they can help smuggle Eren and Historia into Trost for safe harbor with Commander Pyxis.
Keiji is tasked with transporting a covered wagon hiding Eren and Historia to Pyxis while Levi's squad uses decoys to try to carry attention away from them.  En route, Keiji hears gunshots nearby and notices assailants equipped with ODM gear following them. Keiji orders Eren and Historia to stay hidden and attempts to escape. As they are fleeing, Eren and Historia are both knocked unconscious, momentarily distracting Keiji and allowing one of their assailants to kill him.
During the Battle of Shiganshina, as Erwin Smith begins to lament the dwindling hopes of his dream, Keiji stands among the many fallen Scout Regiment comrades in Erwin's mind, wanting to know if their deaths had meaning.
Despite never being seen in actual combat, it is implied that Keiji was a fairly capable fighter, as he survived the 57th Exterior Scouting Mission, during which he directly engaged the Titan horde that was attempting to devour the Female Titan. Furthermore, in the 57th Exterior Scouting Mission, Keiji was part of the team entrusted with guarding the Female Titan. This implies that he was among the few Scout Regiment members who knew about the true objective of the expedition, meaning that he had managed to stay alive for at least the past five years and that he was a trusted member of the Scouts.
- Keiji's appearance underwent an extensive redesign in the second anime recap movie; his brown hair was darkened, his widow's peak and his cheekbones were made less prominent, and streaks of gray were added to his hair to better match his appearance in the manga.
- These changes were retained in future seasons of the TV anime.