In order to draw K. out of his stern studying and humanize him, Sensei brings him into conversations with Okusan and Ojosan. Gradually, K. does open up, but Sensei becomes tormented by jealousy when he happens upon K. and Ojosan talking alone several times.
Over the summer holidays, Sensei and K. leave Tokyo to travel together in the Boshu province. The two remain somewhat distant from each other for most of the trip, but after a visit to the shrine of a famous Buddhist saint, the two have a confrontation, K. arguing strenuously for his ideal of a principled, ascetic life, while Sensei complains that K. is not being human. Following their return to Tokyo, a succession of incidents intensify Sensei's jealousy: he notices K. and Ojosan talking alone in K.'s room more often, sees them walking together outside one day, and finds them teaming up against him during a card game.
Nevertheless, Sensei remains satisfied by his friend's serious and disinterested demeanor that at worst it is only Ojosan who holds affection for K. And so, he is almost paralyzed with shock when K. confesses to him that he is passionately in love with Ojosan. A few days later, while walking in a park, K. brings up the matter again and speaks of his emotional anguish; as Sensei intuits, K. is torn between his strict Buddhist idealism and his love. Fearful of losing Ojosan to K., whom he considers his superior as a rival, Sensei sadistically takes the opportunity to quote K.'s own words from their argument in the summer against him as an accusation of being irresolute and hypocritical. K takes the blow very heavily and mutters some ominous words about his still being able to will himself to do something.
Soon after, thinking that K. meant he was going to pursue his love, Sensei makes a desperate move and asks Okusan to marry Ojosan. Okusan gives her approval, and then Ojosan too consents; however, the matter is not revealed to K. for a few days, and all the while Sensei is tormented by the guilt of having spited his friend and not knowing how to reveal the news to him. One day he discovers that Okusan has already told K., who has not changed visibly. The night before the day he planned to apologize to K., Sensei wakes up to find that his friend has killed himself. In his suicide note, K. explains simply that he had lost hope of attaining his idealistic goals.
Although Sensei does not receive any blame, he feels as though he has killed his friend. His uncle's betrayal made him doubt the rest of mankind, but he had until K.'s death considered himself a virtuous individual; that tragedy made him lose all hope in himself, such that he felt no longer worthy of living. However much he loves Ojosan, whom soon after becomes his wife, Sensei is tormented by the shadow of K.'s death, which he is never able to explain to Ojosan out of fear that it would sully her innocent beauty.
Sensei tries to atone for himself by taking care of Okusan when she is sick and then of his own wife when Okusan dies, but he is nevertheless only prevented from committing suicide out of consideration for his wife. After hearing of the deaths of Emperor Meiji and General Nogi, Sensei finally decides to himself, somewhat as Nogi had done, as a man following the end of his age. He reflects that K. had probably not killed himself out of frustrated love or even the conflict of his ideals, but rather in order to escape a terrible loneliness, which Sensei himself felt.
Returning to the beginning of his letter, Sensei remarks that it was not so much his need to keep his promise to reveal his past to the narrator than his need to tell his story that drove him to write the long testament, and finally asks that the narrator keep the story secret, especially so that his wife will not suffer to hear it.
As much as Sensei seems a place of conflict between different worldviews, the battle is far more intensified in the passionate but strictly governed heart of K. As Sensei analyzes his friend, K's upbringing under the strict moral principles of a Buddhist priest father and then exposure to Western ideas made him into a strenuously idealistic individualist. He says that, "It seems to me that he had more of the priest in him than the average priest" (165-166).
Sensei also writes about K's traditionalism: "You must understand that to K, his own past seemed too sacred a thing to be thrown away like an old suit of clothes. One might say that the past was his life, and to deny it would have meant that his life thus far had been without purpose" (218). It seems to make sense that K took to the Western ideas that conformed to his preexisting Eastern ideas, which created a dangerous mixture. Although the "true way" suggested by ascetic Buddhism would perhaps have just caused him to live a life as a selfless priest or monk, the egoism of Western idealism and its transcendent ambitions created an irresolvable internal contradiction within him which produced, among other issues, the problem of loneliness. It is because of this that Sensei feels himself drawn to and in agreement with K, and then later fearful that he is walking the same path as his friend; we may even say that although Sensei is a typified example of the spiritual conflict of the Meiji era, K is the more extreme manifestation.
Though Sensei usually spoke of humanity in comforting terms, he "marveled at the power of human blood" upon seeing the spurt of blood from K's suicide on the wall (233). This feeling of awe may be likened to Sensei's sense of inferiority in comparison to K: "Where we were in the same class, he was always ahead of me. I had indeed come to regard myself as inferior to K in every way" (177). Although it seems for the most part intellectual, K's love shows him to be more passionate in the raw force of his love and/or in his almost religious resistance to it. The titular theme of the heart is tightly linked to that of blood, and blood is most distinctly manifested in death. Sensei once thought that, "It was my opinion that the silent life K had so far been living had had bad effects on him. I could not help thinking that his heart, like a piece of iron, had gone rusty from disuse," but in a sense the disuse only made it more explosive when it found an outlet in love (178).
In comparison to this simple strength of K's, Sensei is much more crafty and, as he himself admits, weak and cowardly. At one point early on he even asked the narrator whether he thought him a strong or weak man. His sense of inferiority fuels or at least exacerbates his jealousy over Ojosan to the point that it intensifies into fear and desperation. Only then does he gain his desire to harm K and realize how it could be done: "In his innocence, he put himself completely at my mercy. I was allowed to observe him in leisure, and to note carefully his most vulnerable points" (214).
Seemingly contradictorily, this attitude comes from the same person who wrote, "I remember that I used constantly the word 'human' in defending my position and attacking his" (192). The implication is that it is characteristically human to fear and to want to harm others in response to that fear, just as it is to hold concern for others.
Sensei does this by repeating "Anyone who has no spiritual aspirations is an idiot" to K, trying to hurt him by revealing his internal contradiction (214). By his rather strong and inhuman disposition, K neither capitulates away from his position nor becomes angry at Sensei for an obvious act of spite, even when he learns shockingly that Sensei had already become engaged to Ojosan without telling him.
In the end, just before his death, K is described in especially grave imagery, appearing to Sensei several times in the middle of the night as a shadow in the doorway until the night the shadow disappears and is replaced by a corpse; but as K wrote in his suicide note, he was, for all purposes, already dead much earlier on.
Sensei's attitude to Ojosan when she becomes his wife is typified by the experience he had earlier on the rainy day when he sees her walking behind K: "Then I remembered that one of us had to step aside to let the other pass. I moved quickly and stepped into the mud, thus allowing Ojosan to get by" (198). Sensei, feeling stained beyond hope of redemption, fears nothing more than the staining of his wife, and so lives in an idle dead man's life in order to spare her the dirty truth.
He finally turns to death when he feels that it is his time: not so much the death of Emperor Meiji but the death of the Meiji era convinced Sensei that then was the right time to die, and the example of General Nogi gave him the inspiration for suicide, probably also because of the notion that junshi could be an atonement.