Keiji has short brown hair with green eyes. He is of average height with a fair build of a man in the military services.
At the start of the Manga Keiji's lack of experience would cause him to be nervous and he would believe that the first loop was just a dream. However, at his third loop he realizes that it is real. He becomes consumed with fear and tries to escape, even tries to kill himself only to find out that it wouldn't change anything.
Going through 5 loops he comes to the conclusion that he needs to survive the battle and use the loops to train in order to become stronger. This helps him train and become stronger, as he trained every-time and cumulatively strengthened his mind,with each repetition. He no longer comes to fears death but now sees it as his ally in the loop. However, experiencing the same day repeatedly causes Kiriya to lose his interest in many things like food and develop a cold emotionless and depressed expression, due to seeing his friends die every-time which causes him to get into a fight in loop number 80. This leads him to turn down Rachel Kisaragi who offered herself to him before the day of the battle to go and train with the Sergeant who died while saving him the last time in the loop, and because as the way he is right now he can't love anyone as it would go away because of the loop.
Keiji starts as a rookie who has no experience in the battle field. However, due to the repeated experience of combat and training because of the loop Keiji became a formidable experienced soldier capable of easily catching a small piece of falling food with two small sticks while avoiding a hit from a taller and more muscular soldier than him. He was shown to be very observant as he noted the flow of his opponents attack and was shown to be able to "break time" as he was able to react faster than his opponent. On the battlefield the experience and training he went through the loops made him a formidable soldier against the Mimics and was capable of killing many of them in a short time. After being forced to kill Rita in order to escape the loop he was consumed with rage to the point of going on rampage, killing more Mimics in his "first battle" than Rita did in her entire career. This earned him the nickname "Killer Cage" and the fear of his own comrades.
The manga showed that during his childhood Keiji used to play with his friends and pretend that they are fighting aliens. Keiji used to charge against the enemy and "die" which would inspire his comrades and this would help him lead them to charge in order to not let his "death" be in vain.