Kotaru Katsura is a prominent supporting character in Gintama, known for his unwavering commitment to the ideals of the Joui Patriots, a faction of rebels who fought against the alien Amanto invasion that subjugated Japan. He was born into a noble family, but his life changed dramatically when his father was killed under the orders of the Amanto-backed government. This event, combined with the violent oppression of his country, pushed him into a life of revolutionary activism.

In terms of personality, Katsura is calm, serious, and deeply idealistic on the surface. He carries himself with a quiet dignity and a formal, almost old-fashioned way of speaking, often ending his sentences with "ja" or "de gozaru." Despite his serious demeanor, he has a famously absurd and unpredictable sense of humor. He is prone to extreme misunderstandings, takes things too literally, and has a peculiar obsession with the creature "Elizabeth," whom he treats as both a pet and a comrade. He can be stubborn, eccentric, and at times childish, especially when it comes to his favorite foods or his attempts to disguise himself, which are almost always comically ineffective. Yet beneath these quirks lies a core of genuine kindness and a fierce sense of justice.

Katsura’s central motivation is the liberation of Japan from both the Amanto and the puppet shogunate that collaborates with them. Unlike his former comrade Shinsuke Takasugi, who seeks to destroy the world that betrayed him, Katsura pursues a path of more moderate, long-term revolution. He believes in change through strategy, patience, and protecting the innocent, even at great cost to himself. He refuses to sacrifice ordinary citizens for the sake of vengeance, which puts him in direct ideological opposition to more radical factions.

Within the story, Katsura serves as a key figure representing the legacy of the Joui War. He is a terrorist leader in the eyes of the Shinsengumi, frequently wanted on wanted posters, yet he moves about Edo in disguise, planning acts of civil disobedience, sabotage, and guerrilla warfare. His role often oscillates between providing serious historical weight and delivering comedic relief through his absurd escapades. He is also a frequent ally to the Yorozuya, particularly Gintoki Sakata, when a larger threat emerges that requires the strength of their shared past.

Katsura’s relationships are central to his character. His bond with Gintoki Sakata is the most significant. The two were childhood friends, classmates, and brothers-in-arms during the war, with Gintoki as the charismatic prodigy and Katsura as the strategic, earnest comrade. Though they often bicker, insult each other, and lead separate lives, Katsura deeply trusts and respects Gintoki, seeing him as an irreplaceable friend. In contrast, his relationship with Shinsuke Takasugi is tragic; once fellow students under the same mentor, they become mortal enemies after their teacher’s death, with Katsura rejecting Takasugi’s nihilistic violence. He also shares a close, if bizarre, bond with Elizabeth, a silent, placard-wielding creature whose true nature remains ambiguous, but who is his most loyal companion. His interactions with the Shinsengumi, especially Toushirou Hijikata and Sougo Okita, are typically hostile yet sometimes reluctantly cooperative against greater evils.

Over the course of the series, Katsura undergoes subtle but meaningful development. He begins as a somewhat rigid idealist focused purely on overthrowing the government, but he gradually learns to accept that change can come through small human connections and that not all who serve the current system are his enemies. He moves from being a hunted terrorist operating in the shadows to a more visible figure who fights alongside former enemies, like the Shinsengumi, for the sake of the country’s future. His grief over the past—particularly the loss of his mentor Shouyou Yoshida—softens, and he finds a new purpose not just in revolution, but in protecting the fragile peace that Gintoki and others help build.

In terms of notable abilities, Katsura is an exceptionally skilled swordsman, ranking among the strongest characters in the series. He was a prodigy of the Joui War, known as one of the "Four Devas" alongside Gintoki, Takasugi, and Tatsuma Sakamoto. His fighting style is calm, precise, and adaptable, capable of taking on multiple opponents or powerful Amanto. He is also highly intelligent, possessing a sharp strategic mind for planning rebellions, escaping capture, and analyzing situations. Beyond the sword, he is proficient in using explosives, covert operations, and various forms of disguise, though his disguises are often comically transparent. He also has surprising physical endurance and resilience, able to survive injuries and explosions that would fell most others. His most unconventional "ability" might be his baffling capacity to misinterpret normal situations in the most surreal way possible, a trait that often lands him in comic trouble.