Though outwardly brash, haughty, and arrogant, Kaigaku is in fact cowardly and selfish. Like many orphans who grew up with nothing, he resorted to doing any immoral or demeaning acts as a means to survive, like thievery and drinking from a puddle.[4][3] His intense sense of self-preservation goes so far as to sacrifice the lives of Gyomei Himejima and the other children to save his own skin, showing that he will do anything if it means survival. This attitude also was evident during his time as a Demon Slayer; unlike those who join the Demon Slayer Corps, Kaigaku never developed a desire to slay demons or protect people. When confronted with a stronger opponent, Kaigaku immediately surrenders, once again cementing his cowardice and indifference towards making immoral choices if it will save him in the end.[3] However, despite his disinterest in becoming a Demon Slayer for the greater good, Kaigaku did show a desire for a status as a master swordsman. This is shown when even as a demon, he chooses to make his flesh weapon of choice resemble a Nichirin Sword.

Kaigaku has a sense of hypocritical self-righteousness. He considered Zenitsu worthless for his perpetual cowardice and inability to master the Thunder Breathing forms beyond the first; for these reasons, he was repulsed at the idea of succeeding Jigoro along with him. However, as he could not master the first and fundamental form himself, his actions proved him to be just as cowardly as Zenitsu, if not worse, and his accusation of Zenitsu shaming Jigoro proved void when he himself ends up disgracing the former Hashira by becoming a demon. Furthermore, when confronted with the news of Jigoro's death, he only claimed his demise was earned for not believing he alone was worthy to succeed hiKaigaku also possesses a twisted perspective of "right" and "wrong", believing that those who judge him right (i.e., say he is worthy) are good and those who judge him wrong and have a low estimation of him are evil.[5] His inflated ego and superiority complex was evident when could not stand the idea of sharing the position of Thunder Hashira with Zenitsu, and later accused Jigoro of favoring Zenitsu over him. In addition, when Zenitsu points out how Kaigaku could never master the first form and remarks that he pitied Jigoro for having such pathetic successors, Kaigaku immediately lost his composure and screamed at Zenitsu to not compare them to each other.

His myopic view of victory was such that if one hasn't died, even if they've suffered through hardships such as losing their home or drinking dirty water, or if they resorted to committing petty acts such as stealing money to survive, they haven't lost and can come back for victory someday.[3] As a result, he deluded himself into believing he was the winner of his fight with Zenitsu simply because the latter was going to fall to his death,[6] even though they both were going to die in that moment and Zenitsu was for all intents and purposes the true victor, having beheaded Kaigaku before he could react and initiating his imminent demisWhile he was already spiteful and cruel as a human, as evidenced by his constant verbal abuse of Zenitsu, becoming a demon revealed a much more unrestrained and psychotic side to Kaigaku. Not only was he gleeful at the prospect of killing the junior he had long hated with his newfound power, but he expressed his satisfaction towards Jigoro's with a large smile on his face, and referred to his former master with insulting labels like "old fart".

Kaigaku served as a foil to Zenitsu; while Zenitsu was a defeatist who was at least self-aware of his shortcomings and desperately wanted to improve upon himself, Kaigaku was a determined yet conceited egoist who barely reflected on his own cowardice, misdeeds, or misfortunes, regarding them as meaningless past actions he committed to survive. In addition, while Zenitsu found companionship through Tanjiro Kamado, Nezuko Kamado, and Inosuke Hashibira, Kaigaku was seemingly alone, with other Demon Slayers disliking his arrogance.

In spite of all his flaws, Kaigaku seemed to be genuinely happy as a young child living with Gyomei and other orphans, cheerfully conversing and eating with them, and smiling when being pat on the head by Gyomei. However, his egocentric nature outweighed his attachment to his first found family. In the end, after being beheaded and being at death's door, Kaigaku remains unrepentant and is pleased to know that at least Zenitsu would die with him. However, Yushiro arrives and begins mocking him that he will be given nothing since he never learned how to give and will now die alone; Kaigaku's fate was sealed once he decided to let his desire inflate and take control of him. Upon seeing Yushiro rescue Zenitsu, Kaigaku screams in anger as the remainder of his body disintegrate.