Quitting won't do you any good; you have to work for an answer.
~ Kenji Koiso

Kenji Koiso is the main protagonist of the 2009 anime film Summer Wars. He is a 17-year-old student at Kuonji High School and a math genius. One day, he is invited by his friend Natsuki Shinohara to her great-grandmother's 90th birthday where, much to his shock, he is to pretend to be her boyfriend. As he struggles to fit in, he receives an e-mail with a mathematical problem. After he cracks it, he discovers that his account has been hacked by a mysterious AI named Love Machine. Before he knows it, Kenji, Natsuki and her entire family are caught up in a battle that will determine the fate of the entire world.

He is voiced by Ryunosuke Kamiki in the Japanese version and Michael Sinterklaas in the English version, both of whom also voice Taki Tachibana.


Kenji appears to be of average height for his age, being slightly taller than Natsuki. He has brown eyes and straight brown hair and a fairly delicate expression. He is usually shown wearing a simple blue shirt.


N-n-n-n-n-no! Absolute not. Natsuki, I've never even asked a girl out on a date before.
~ Kenji objecting to pretending to be Natsuki's boyfriend, highlighting his awkwardness

Kenji is shown throughout the film to be shy, awkward and easily flustered; upon learning that he has to pretend to be Natsuki's fiancee for the duration of the family reunion, he is initially strongly against it, despite having a secret crush on her, as he had never even asked a girl out on a date before. Despite his limited social skills, he is shown to be a very kind and polite young man, as he never loses his temper with any of the Jinnouchi family and even after he gets blamed for the Love Machine's outbreak, he thanks them for letting him stay at their house. He claims that he rarely gets to have dinner with anyone since his parents are often a work, implying that he is rather lonely.

Kenji is a maths wizard, being able to solve complicated problems very easily and even working as a part-time moderator in the massive computer-simulated virtual reality world OZ. It gets to the point where solving maths problems is a hobby of his, to the point that he can't resist solving a problem whenever he's presented with one. Sadly, this ends up being his undoing when he cracks a mathematical code that enables Love Machine to hack his account, and use it to hack OZ's infrastructure.

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