Ishigami has claimed to be a stalker suffering from delusions of being Yasuko’s bodyguard. While this is certainly a calculated ruse meant to mislead the police, it is also very true on an emotional and figurative level. Ishigami really did come to Yasuko’s aid in the hopes of protecting her. He really did see her everyday at the lunch-box restaurant in order to satisfy his longings to be near her. And he really did leave a threatening letter for Kudo, demanding that Kudo cease his advances on Yasuko. In some sense, the "stalker" story, while totally contrived for the purpose of outwitting the police, is entirely true, and yet it becomes justified as an elaborate plot developed in a spirit of self-sacrifice.

Up until now, the reader has felt ambiguity about Ishigami, especially given how Ishigami behaves towards Kudo and also how Yasuko worries about the emotional blackmail aspect of Ishigami’s being privy to her misdeeds. In this way, the ambiguity surrounding Ishigami is resolved: he has embraced his role as a stalker but done so in a way that is paradoxically self-sacrificial and heroic.

Ishigami becomes a heroic stalker, as it were, and the tension underlying this duality causes the reader to have mixed feelings about him. This duality evokes a sense of mono no aware, a bittersweet nostalgia defined by admiration for Ishigami's heroic devotion to Yasuko coupled with uneasiness about his uninvited, possessive, and insecurity-driven intrusion into her life. While many characters in the story are undeveloped two-dimensional stereotypes, acting as foils or serving to move the plot in a certain direction, the arc of Ishigami’s character development is a tour-de-force and one of the greatest parts of the novel. Keigo Higashino’s has really outdone himself in the character of Ishigami.

According to some interpretations of existentialism, it is said that “existence precedes essence.” Ishigami’s actions up until now have been ad hoc (albeit well-planned) ways of dealing with Yasuko’s circumstances, driven by a kind of mathematical necessity and fatalistic inevitability. His attraction for Yasuko is mostly driven by lust and primal instinct rather than anything transcending biological necessity. In the end, though, he consciously accepts all of the insecurities and weaknesses that drove many of his actions and embraces the role of a stalker and thereby becomes a hero. In life, all too many circumstances are beyond our control – it can be said that we are thrown into life against our will and that this throwness is what defines the human condition. But we don’t have to be helpless victims of fate. By choosing to embrace the identity that is often thrust upon us by circumstance, we can transcend those circumstances and attain a degree of freedom to define who we are. In this sense, Ishigami is very much an existentialist protagonist who creates his identity and defines the meaning of his life.

Lacking emotion and feelings, much like the main character in Camu’s The Stranger, Ishigami is a person who is mostly indifferent to life. He doesn’t feel the emotions society expects “normal” people to feel under various circumstances. For example, he doesn’t feel compassion for the homeless people whom he notices on his way to work, nor does he show any interest in getting to know them as individuals – rather, he sees them as part of the “clockwork” of the mathematical order of the universe and gives them superficial and purely functional names, like the “Can Man” or “Engineer.” He is even willing to use one of them as a tools for completing his plan, which involves cold-blooded murder. He doesn’t feel empathy for his failing students, whom he burdens with perpetual make-up exams despite the fact that the school will not allow them to get a failing grade. He can’t understand why his students have such little passion for mathematics, nor can he understand how they fail to grasp the problems which he unwittingly makes too complicated. While mathematics is irrelevant for the future career goals and other life aspirations of his students, for Ishigami, mathematics is all-pervasive and applicable to every aspect of life. Moreover, his perpetually blank expression is totally devoid of emotion and unsettling for everyone who happens to talk with him. There is no question that he is a Camuesque figure - a person buffeted by circumstances and whose actions are logically and metaphysically prior to his emotions.

Another figure that comes to mind when considering Ishigami is Dostoyevsky's Raskolnikov in Crime and Punishment. Like Raskolnikov, Ishigami attempts to become a Nietzchean ubermensch by outwitting the police and formulating the perfect cover-up - an unsolvable mathematical problem from the perspective of the detectives and a perfect solution to the problem of Yasuko's circumstances from the perspective of Ishigami. Throughout the novel, Ishigami frequently attempts to push the bounds of what is possible in mathematics. He tries to find proofs for problems that other have failed to solve. It is quite appropriate that his classmates dub him " The Buddha," since his single-minded contemplation of mathematical problems enables him to transcend many of the concerns, anxieties, and weaknesses of most humans: he doesn't mind losing opportunities for working in a university, nor does he mind even being in prison. As long as he is allowed to work on math problems, he is perfectly content. He has achieved an liberation from the cycle of birth, death, and suffering and pierced the veil of maya, seeing beyond the illusory nature of this world and its seemingly unsolvable problems and dilemmas to discover the real substance that makes solutions possible. Like Raskolnikov, he commits an actual murder, and he does it in order to carry to completion a master plan that will vindicate his desire for logical perfection. Ultimately, however, he fails because of his inability to understand the sentimentality and emotions of real human beings.

At the same time, while being an existentialist protagonist who is able to define his identity, the final scene of the book shows him to be a tragic figure as well. He sets in motion a plan that is intended to bring about the salvation of someone whom he loves (though it is a primitve love based on simple biological instinct). But in carrying out his grand plan, he ends up destroying the object of his affection and causing his own downfall. His sadness at the end of the novel conveys grief that can be understood on multiple levels. It is a grief arising from the failure to solve a complex mathematical problem for which he had formulated an iron-clad solution. The very basis of his identity, which is his ability to solve complex problems, is severely undermined by his failure to bring his solution to fruition. It is a grief arising from the harm done to someone for whom he feels affection, though it is a selfish and primitive affection in that Yasuko is only important insofar as she helps him justify his own existence and avoid committing suicide. It is also grief based on an inability to defeat fate and is expressive of the helplessness of the human condition. For all these reasons, it is a grief that induces mono no aware in the reader: we feel pity for Ishigami, knowing the extent of his sacrifice, yet we also feel put off by the somewhat superficial nature of his attraction to Yasuko, the insecurity and jealousy underlying many of his actions, and the self-serving and dispassionate manner in which he uses people to help satisfy his purely intellectual curiosity for mathematical phenomena.