Born Tsukioka Katsuhiro, the man known in the Meiji era as Tsunan is a prolific nishiki-e painter in Tokyo, known for his beautiful inkings of popular historical and cultural figures. However, Tsunan is secretly a survivor of the Sekihō Army like Sanosuke and has spent the last ten years plotting revenge against the Meiji Government.
- Sagara Sōzō - Mentor and leader during his time as a member of the Sekihotai.
- Sagara Sanosuke - Best friend since childhood.
Katsuhiko is skilled at making and using bombs. He wears rings on his index and middle fingers that create a spark when they snap against one another to light his bombs quickly.
In his youth during the Bakumatsu, Tsukioka Katsuhiro was a junior member of the Sekihō Army along with his young friend Higashidani Sanosuke. Looking upon First Squad Captain Sagara Sōzō as a hero and mentor, Katsu proved his usefulness by skillfully assembling rifles and expertly mixing gunpowder for the militia's soldiers. However, when the Sekihō Army was disbanded and marked as frauds by the Meiji Government and Captain Sagara was beheaded and put on public display, Katsuhiro fell into a deep, angry depression and spent the next ten years of his life in solitude. Moving to Tokyo, he changed his name to Tsukioka Tsunan and began using his steady hands to create nishiki-e paintings in order to support himself. With his skills, Tsunan became a rather popular artist, making a steady and sizeable profit off his paintings of popular bakumatsu figures like Iba Hachiro. He also attempted to sell several paintings of Captain Sagara, but to no avail due to the Sekihō Army's infamy.
However, Katsu's true objective since the end of the Sekihō Army has been avenging the captain's death. Utilizing his expertise with gunpowder, he has spent ten years in seclusion constructing a large stockpile of bombs with the intention of demolishing the various centers of government around Tokyo and inciting a revolt among dissatisfied farmers around the nation. In the early Spring of 1878, just as Katsu had finished preparing his explosives and had mapped out his targets, he was greeted by Sanosuke, who had learned of his presence through a painting of Captain Sagara.
Taking the appearance of his old friend as a sign, Katsu explained his true intent to Sano and described his plan: on the next night, a Sunday night with a new moon, he would start his bombing campaign at the Department of Internal Affairs, then move on to the Military Headquarters and the Ministry of Finance; with these centers of power demolished, the oppressed farmers and samurai around the nation will see their chance to revolt and the Meiji Government - still weak from the expenditures of the recent Seinan War - would lack the power to stop them. Katsuhiro asks Sano to join him in crushing the Meiji and bringing about the realization of Captain Sagara's dream of a truly equal Japan.
Sano agrees on the condition that Katsuhiro help him throw one last party for all his friends at the Kamiya dojo. Katsu complies and, once the party is over and everyone else has fallen drunk, the two remnants of the Sekihō Army head out for the Department of Internal Affairs. At first, it appears that their plan is going perfectly, but before they can enter the government building, they are stopped by a man who Katsu recognizes as one of the guests at Sano's party - a man Sanosuke identifies as Himura Kenshin, the legendary Hitokiri Battōsai. Though Katsuhiro pelts Battōsai with bombs, Kenshin disarms each of them with his sword. Before Katsu can decide on his next move, Sano explains that they can't get past and swiftly punches Tsukioka in the stomach, knocking him unconscious.
When Katsuhiro awakes the next morning, he finds himself in his apartment with Sano sitting nearby and all of the bombs gone. Sanosuke explains that Kenshin took all the bombs and buried them somewhere secret. Sanosuke explains that, like Captain Sagara, the Battōsai wouldn't want them dirtying their hands to accomplish good, but Katsu responds that no deed is too dirty if their goal is achieved. As Sanosuke leaves, he remarks that such thinking will make them into exactly what the government called them - a fake revolutionary army. Rather than sinking to the level of villains, Sano claims that he is satisfied to change the world slowly and steadily, making Captain Sagara smile from the afterlife.
Tsukioka's plot plays out somewhat differently in the anime adaptation. Instead of turning on Tsukioka, Sano challenges Kenshin to a rematch in order to buy Tsukioka time to complete their mission. Despite his hatred for the Meiji, Tsukioka cannot bear to abandon his friend to certain defeat and pleads with Sano to fall back. Sanosuke however continues his assault and, though he fights well, is knocked unconscious. Kenshin leaves Sano in Tsukioka's care and does not defend himself or his actions when Tsukioka calls him a hypocrite for defending the Meiji Government. Sanosuke then stirs and thanks Kenshin for stopping the Sekihō Army from truly becoming a false army, a sentiment which shocks Tsukioka to his core. After patching up Sanosuke, in the morning Tsukioka buries the remaining bombs himself.
In either case, Tsukioka takes Sanosuke's words to heart and decides to give up his plans of terrorism as well as his nishiki-e career in order to self-publish a liberal newspaper publicizing the Meiji government's wrongdoings. But before his quits, Tsukioka Tsunan paints one final work of Sagara Sōzō specifically for Sanosuke.
Tsunan reappears in Act 58, when Sanosuke comes to him asking for money to fund a trip to Kyoto. Though Sanosuke doesn't explain the excursion, Katsu reveals that his contacts in journalism have already informed him of Shishio Makoto's actions in the old capital. Having developed powerful but inconspicuous new bombs (for self-defense), he gives several of them to Sano for his friend's protection. Sanosuke asks Katsu not to publish anything about Shishio's army for his own safety, but Tsunan responds that he hadn't intended to anyway. However, as Sanosuke prepares to leave, it becomes apparent that he was followed to Tsukioka's residence by Myōjin Yahiko and also Saitō Hajime. As Saitō tries to convince Sano not to go to Kyoto, since his presence would only be a hindrance in the battle against Shishio, the two men engage in a fistfight. Though Saitō demonstrates that his level of fighting prowess is much higher than his, Sano manages to prove that even his strength is too significant to ignore. Katsu heals his friend's wounds after Saitō departs.
According to Watsuki, Tsukioka Tsunan's character began simply as an idea for a explosives expert and ex-comrade of Sanosuke's for a trio of chapters in a bonus manga arc which would precede the Kyoto Arc's start. Having no time to design a brand new character with the massive Kyoto project looming, Watsuki claims to have simply taken Tsukioka's design from an old sketch he'd made in his notebook -- a dreadlocked character whom Watsuki had thought might make a decent villain some day. With a few tweaks, the character's basic appearance came to resemble the current Tsukioka, but after coming to the conclusion that having the character work under cover as a "fake artist" would make him more interesting, Watsuki decided to add "artist-y" elements such as an oddly-patterned jacket and a bandana (disregarding whether or not bandanas were in style during the Meiji Era).