Bansai Kawakami is a member of the Kiheitai, an extremist Joui faction led by Shinsuke Takasugi. He serves as the group's de facto second-in-command and chief negotiator, often handling delicate dealings with other organizations such as the Harusame space pirates. His background is largely shrouded in mystery, but he leads a notable double life as a successful music producer for the popular idol Terakado Tsuu, using the pseudonym Tsunpo to conceal his identity as a wanted criminal.

In terms of appearance, Bansai has a modern and fashionable style. He sports styled teal-green hair, a matching tight coat, and is never seen without his signature pair of sunglasses and headphones, which feature distinctive yin-yang symbols on the sides. He always carries a shamisen, a traditional Japanese stringed instrument, which serves multiple purposes; it is a weapon in itself, its steel-like strings can be used to ensnare and cut opponents, and it also hides a sword within its neck.

Bansai's personality is a blend of refined composure and hidden viciousness. He is a polished and silver-tongued speaker who uses an archaic and polite form of Japanese, which contrasts sharply with his cruel and ruthless actions as an assassin. His constant wearing of headphones contributes to an air of detachment, earning him the nickname Deaf Man, though he seems to have a unique ability to listen to the "tune" or rhythm of a person's soul, which dictates his interest in them. This sensitivity to the "beat" of a person's spirit is central to his character; he is a man who follows his own aesthetic sense and will back out of a situation if he cannot get into its rhythm, even if it means disobeying an order from Takasugi. He is also shown to be extremely literal, once taking a sarcastic joke from Sakata Gintoki about finger beam attacks seriously and preparing to sever his own finger.

Bansai's primary motivation is not political idealism but a deep-seated fascination with the music created by Takasugi Shinsuke. He believes Takasugi will compose a beautiful and world-changing opus, and Bansai has attached himself to the man's cause to witness and facilitate this transformation, even if it means bringing about destruction. While he shares Takasugi's goal of tearing down the corrupt Bakufu and the Amanto, his loyalty is ultimately to the "song" of his leader and the aesthetic of their shared path. He is not an unquestioning follower, however, and has shown the ability to relate to the motivations of enemies like Gintoki, understanding their desire to protect what is precious.

In the story, Bansai plays a significant role as a major antagonist, most notably in the Shinsengumi Crisis Arc where he masterminds a plot from within the Shinsengumi by manipulating the ambitious officer Kamotarou Itou. His role evolves over the series; after a climactic defeat by Sakata Gintoki, where he admits to being captivated by Gintoki's rhythm, he becomes less of a pure villain. He later participates in the conflict on Rakuyou, fighting alongside former enemies like Gintoki and Katsura to confront a common threat. His journey culminates in a heroic sacrifice during the Silver Soul Arc, where he detonates a bomb on his own body to buy time for his comrades, dying with a smile and finally fulfilling his role as one who chases after Takasugi's back.

His key relationships are deeply meaningful. His bond with Takasugi Shinsuke is the most important, and he is the only Kiheitai member who refers to him casually by his first name. They share a deep mutual respect, even playing a shamisen duet together. He also has a caring, almost senpai-like relationship with his fellow Kiheitai member Kijima Matako, protecting her on multiple occasions. His relationship with his enemy, Sakata Gintoki, is complex; it begins with antagonism but grows into mutual respect, as Bansai comes to admire the "song" of Gintoki's unwavering soul.

Throughout the series, Bansai demonstrates notable development. He transitions from a mysterious and ruthless antagonist who manipulates others for destruction to a more sympathetic ally who fights alongside protagonists and makes a noble sacrifice. His respect for strong spirits, first shown when he spares the Shinsengumi's Yamazaki Sagaru to hear the continuation of his song, becomes a recurring theme that defines his actions.

Bansai's abilities are formidable. He is an extraordinarily skilled swordsman capable of holding his own against Gintoki, one of the series' strongest characters. His unique fighting style incorporates the strings of his shamisen, which he can use to restrain opponents from a distance with incredible precision and strength. His ability to sense the rhythm of his opponent is a form of combat precognition, allowing him to predict and counter attacks. Beyond his martial prowess, his intelligence and eloquence make him a master manipulator and a vital asset in negotiations, capable of swaying powerful groups like the Harusame to his side. His love for music is not just a hobby but the very lens through which he views the world and its conflicts.