—Okay, scratch that, no. More like roughly 20 years ago in a fictional city called Edo, Japan, was taken over by aliens called Amanto as they were beginning to come out of isolation. You know, these cool samurai series now that take place during the historical Tokugawa period. That's what Gintama is. Samurai series with a bunch of modern and futuristic bullshit all rolled into one. Anything and everything goes from flying space ships, to videogame devices similar to the Wii, to loyal samurais who are still holding on to their swords despite there now being a ban to use them.

Basically, sci-fi samurai hell with copious amounts of ball jokes.

In any case, government and political officials gave into the aliens' leadership out of fear for their own lives, and then found their own benefits in it. The samurais that flourished in the country refused to give in so easily, having tension with these intruders throughout the years. They want to regain their pride as samurai and ward the Amanto out of Edo. One of these samurais was Katsura Kotarou, and his training began at a young age.

We do know he studied under Shouyou-sensei's teachings, along with his classmates: Sakata Gintoki and Shinsuke Takasugi. They all learned together to be strong samurais under a shelter / school that Shouyou taught. He was a kind man and often took in children who couldn't afford school to teach them things without any fees. It took awhile for them all three of them to study under the same roof as Katsura and Takasugi were studying at a different school at the time, but eventually they all were united.
When they had heard rumors of a temple school that was taking in poor children and teaching them, they began to investigate. It would be worth it; especially since Katsura and Takasugi were exhausted with their pressure-free an otherwise boring education. The teacher was famously known for adopting a silver haired boy named Gintoki and raising him by the way of the sword. Eventually, they found out that the school was also a place for children to gather and criticize the newly founded Amanto-based government, along with them training to be samurais, which was also a big no-no. Everyday, Takasugi would go by Shouka Sonjuku, Shouyou-sensei's school, in attempt to beat Gintoki. Takasugi would end up defeated and starved by his family as discipline, told he would be disowned if he attended the school again. Even as youngsters, Katsura's heart was large. He was always there to converse with Takasugi and give him food in secret.

Eventually, government officials learn what Shouyou's school is teaching, and plan to destroy it. That night, Takasugi and Katsura decide to defend the school, and Zura even warns the school before hand of what is to become of the school. They do this even though they are aware they would lose their current scholarships and be disowned. Still, Gintoki and Shouyou show up, and Takasugi and Katsura decide to become his students instead as they all defend their school.

Fast forward some years later, when the samurais really begin to fight back against the Amanto and ward them out of Edo, a specific war was fought between Amanto and humans called the Joui War. When Shouyou's students discover that their teacher had been arrested, his students picked up the sword and joined the losing war against the Amanto. Around this time is when Katsura was introduced to Sakamoto Tatsuma, the 4th samurai and last part of the Joui Four that will go down in history despite them joining towards the end of the war.
When surrounded by the enemy, Katsura, who is very much accustomed to old fashioned samurai ways, suggested that Gintoki and he commit seppuku and die beautifully with honor. Gintoki comes back with they should live beautifully instead. This encourages Katsura to live on and fight and win instead of fruitlessly die trying. Even though they won that singular battle, Shouyou was not rescued and was beheaded. Gintoki, unfortunately, was the one to do the honors and behead his own teacher against his will. The enemy gave him an option: behead your teacher to save your friends or they will be the ones who perish. Gintoki chose to save his friends.

Everybody seemed to split as time progressed, going their separate ways. The war was lost. Gintoki decided to stay put in Edo after the war, Sakamoto went to space claiming he was tired of seeing his comrades being murdered. Katsura and Takasugi went separate ways, still despising the Bakufu government. They both serve vital roles in their responsible anti-government organizations and continued to pursue the war against them through underground methods. Takasugi's ways have been and always will be violent, in shock and bitter over the loss of his teacher; he still sees his world as nothing now; an imperfect mess that should be destroyed and reconstructed for taking away his mentor. Katsura's organization is a little different.

Early in the series, Katsura seems to still be very passionate about destroying the Bakufu government and those who side with it. He uses violent and extremist methods much like Takasugi, specifically using schemes that involve his infamous weapon: a metallic bomb with a timer. Gradually, Katsura begins to understand that extremest ways aren't the best solutions to resolve things and merciless bloodshed should stop. You could say he becomes a somewhat pacifist after time.  This is a process, mind you. Katsura is stubborn as an ox when it comes to radical changes. One of the key things to make him change his heart is a love interest, accompanied by the words of others.

The woman's name who becomes his love interest is Ikumatsu, a widow girl who owns a noodle shop and allows Katsura to hide out and heal from an injury caused by the Shinsengumi in her restaurant. She is unaware of his identity and heals him back to strength, though she mentions that her husband was killed by Joui Patriots and despises them. Still, she continues to care for Katsura, and this begins to stir his heart more so than any of Gintoki speeches on why he needs to stop doing what he is doing. Even when Katsura's identity is revealed, they still remain on good terms. If she can go on and endure her pain so freely— enough to still remain friends with somebody who associates with a group that took her husband away— surely Katsura can find better means to go about seeking satisfaction.

Another instance in which Katsura has realized that he has no reason to hate the world is in the Benizakura Arc when he confronts Takasugi. He explains that while Gintoki has every reason to hate the world that they live in more than them, he endures it. He has found things in Edo that he treasures, and he also mentions that destroying Edo would not have been what Shouyou would have wanted.

This gives him hope in other things that reside in Edo.

Even so, Katsura wishes for things to change back to their old ways throughout the series through political activism. He once has plans on destroying a restaurant owned by completely Amanto citizens by use of terrorism, but later learns through Prince Hata who is also working there at the time that there should not be any bloodshed anymore. If everybody lived by things such as love and peace then they would be happy. Katsura takes this to heart and changes his mind about destroying the restaurant and instead defends the restaurant when a group of radicals plan on destroying it during his time there.

It seems he will continue to find different ways to try and change Edo back to the way things were, all the while trying to convince others to join his organization and avoid the police for crimes committed against them. He's seen throughout the series still running from the police or doing harmless things against the Bakufu and Shinsengumi, though he would have no qualms severely injuring them still.

As the series continues on, Katsura seems to get involved in other crazy antics in various arcs, specifically if the members of Yorozuya are around.