Kisaragi Saemon is a ninja from the Kouga Manjidani clan who is selected as one of its ten representatives in a deadly supernatural conflict against the rival Iga clan. He is the older brother of fellow Kouga ninja Okoi, and his love for her is the most significant emotional connection in his life, marking the one relationship where his typically detached demeanor gives way to genuine feeling. In the original novel, Saemon is described as having a completely featureless and ordinary appearance, so plain that a person might forget what he looked like moments after meeting him, leading to speculation that even his everyday face might be a form of disguise.

Saemon is often considered one of the most complex characters in the story, maintaining a cool, aloof, and almost professionally detached conduct in both times of war and peace. He is generally quiet and laid-back, speaking in a calm undertone that can augment an apathetic presence, and his narrow eyes typically remain half-closed, opening fully only when his emotions are stirred, particularly by anger or the conception of a cunning plan. Despite this mysterious and sometimes menacing exterior, he can be generous and is perceptive, keeping an observant eye on his comrades and understanding their hidden feelings, such as being aware of Kagerou's love for their leader Gennosuke. He is steadfast in his commitment to success and holds a rigid view of the war, believing that any method is necessary to secure victory for his clan.

His motivations are fundamentally altered by the brutal death of his sister Okoi at the hands of the Iga clan. Initially, he does not pay much attention to the conflict, but after witnessing Okoi's fatal wounds and holding her hand as she dies, he throws himself wholeheartedly into the war, viewing the absolute victory of the Kouga and his personal revenge against the Iga as one and the same. To achieve this, he becomes willing to physically and mentally break his enemies, using any means necessary, including psychological warfare and cruel taunts. However, a distinctive aspect of his character is that after he kills an opponent, he will always apologize to them, expressing a hope that in death they might find a peace that was denied to them in life, revealing a remnant of his humanity beneath his ruthless actions.

Saemon's role in the narrative is primarily that of a master strategist and infiltrator, using his exceptional disguise abilities to create chaos and eliminate key targets to support his allies rather than engaging in direct, head-on combat. He forms effective partnerships with his fellow Kouga ninja, particularly Gyoubu Kasumi, with whom he works seamlessly, and Kagerou, with whom he shares a strong bond and a successful collaboration in eliminating multiple Iga members. His most notable antagonistic relationships are with Hotarubi, whom he kills after using the face of her dead lover Yashamaru to deceive her, and with Nenki Mino, whose corpse he desecrates after Nenki is killed by another ally, as revenge for Nenki's role in Okoi's death. His development throughout the story shows a transformation from a relaxed individual into a focused, vindictive instrument of revenge, though careful observation reveals the personal cost and inner struggle he endures as he commits terrible acts in the name of his mission and his sister.

Saemon's most notable abilities are his mastery of disguise, which operates through two complementary techniques. The first is the ability to mimic any voice he has heard, though this mimicry can fail if he is distracted or shocked. The second and more distinctive is his method of physical transformation, where he presses a person's face into soft mud or clay to create a mold, then presses his own face into that mold to perfectly replicate their appearance. This technique is only cosmetic and does not grant him the target's memories, body proportions, or unique ninja abilities, making it less effective if the person is of a significantly different size. Furthermore, while it can fool humans, animals or creatures with superior senses, such as Hotarubi's snake, can see through the illusion. Despite these limitations, Saemon is a highly skilled weaver of deception, adept at crafting convincing lies and maintaining a persona without arousing suspicion. In addition to his infiltration skills, he is one of the Kouga's most capable hand-to-hand combatants, having demonstrated precise and lethal efficiency against multiple armed opponents and defending himself against skilled attackers.