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Aoshi Shinomori was picked at an early age (15, to be exact) to become the leader (or more commonly called "Okashira") of Japan's most esteemed assassins group, The Oniwaban-shu, literally the "Guardians of the Garden". He was known for his sword skill throughout the lands he was in, and was also noted greatly for his speed and skill with a Kodachi (which is, a japanese short sword/katana), and for being very handsome.
Throughout his time as the youngest assassin in the lands and during the time of the Bakumatsu Era, he strove to be the best swordsman of the time, and to be better than the legendary Hitokiri Battousai, also known as Kenshin Himura.
Aoshi's Oniwaban-Shu group was much differently formed than the one that Kenshin met later in the series in the Kyoto Arc, mainly due to the fact that they didnt try to kill him, they moreso gave him a home to live in while he stayed there attempting to defeat Shishio, a mad man also from the Bakumatsu Arc who lived in the Battousai's shadow after the legendary "man slayer" had laid his sword to rest and vowed never to kill again. Aoshi's Oniwaban-Shu was formed with the intent of ultimately becoming the greatest in Japan, intent on defeating Kenshin before anyone else could, and so, he was more of a mercenary than an assassin with hopes of bringing peace in their currently corrupt world.
In the Series
Aoshi's first appearance is in Rurouni Kenshin volume Four, in which he is on the cover of the graphic novel along with Kenshin, Sanosuke, and Yahiko.
Aoshi, with the assistance of his friends that he'd formed to make the Oniwaban-shu worked under a malicious and greedy man going by the name of Takeda Kanryu. Kanryu worked in the black market business, making opium and illegal weapons for the Chinese Mafia, or anyone that paid enough. Aoshi was only being his body guard with intent of meeting Kenshin in the area, as he knew that the former battousai was attempting to redeem himself, and would eventually try to stop an illegal act like this, especially if their was a woman involved. This woman had requested for Kenshin's help after she had barged into one of his and Sanosuke's gambling games, with Kenshin only doing it for fun, and Sanosuke aiming for profit.
Kenshin saw the two men chasing her, and stopped them both, and they decided to assist this woman, who had obviously been being hunted by these villainous characters for some time. She stayed at the Kamiya Kasshin Ryu dojo for several days before the men of the Oniwaban-Shu had come after her, only to be defeated by Kenshin and friends. Eventually, Megumi went back to Kanryu, after he threatened to kill the people she'd been staying with, and since she couldn't bare with the guilt, she decided to turn back.
Eventually this lead Kenshin straight into Kanryu's mansion like home, and got Aoshi what he had been doing this for all along: Killing Kenshin. Aoshi proved to be Kenshin's strongest opponent yet, with speed almost matching his own. In fact, throughout the fight, Aoshi was moving so fast that Kaoru, Sanosuke, Megumi, and Yahiko, could barely catch the duplicates of Aoshi surrounding their friend in the duel. Aoshi proved that he not only was a skilled swords man, but he was also an expert in physical close range combat, giving Kenshin bruises left and right during the fight through punches flurrying so fast that their eyes could not catch them.
Aoshi eventually was struck in the throat by Kenshin's reverse edged blade, and Aoshi was forced to stop momentarily. He gave the match up after Kenshin told him that even he could not take two blows to the throat. Aoshi wasn't truly prepared to give up his lust for the title, but their fight was then interrupted by Takeda entering the giant ball room with a Gatling gun. Aoshi's legs were shot with a fusillade of bullets, and with a look of shock, he seemingly passed out. Kenshin was in no state to face the gun shots, and could not fast enough, but one by one, Aoshi's friends that he had formed in the group were taken down by the bullets, all protecting him in some way, redeeming almost everything the series so far had done to make them seem evil. The muscle bound opponent of Sanosuke blocked the bullets head on, noting that his body had been useful for something at least once, and then Han-Ya had used his martial art skill that Aoshi had taught him to stop some of the bullets and move
Kenshin out of the way, and the small skinny warrior skilled with darts and other projectiles was able to throw one of his small bullets into the row of ammunition of the Gatling Gun. Kanryu then went unconscious after taking an aggressive blow to the face by and infuriated Kenshin, as he was asked where all his money as now to save him.
Heads of his Partners
The police busted into the mansion, and arrested the drug lord and weapons dealer. They went into the giant ball room to find Aoshi and co, only to see that he was gone, and all of his friends heads were gone. (It should be noted that in the anime, being a show for younger audiences also, that they say he just took their bodies with him. In the original manga, he decapitated their bodies to carry with him to go bury them).
Kenshin, after rushing outside, was met by Aoshi promising to avenge his friends deaths, and one day defeat Kenshin for title of the strongest once more. After one year had passed from their fight, Kenshin was asked by Lord Okubo, an ex-military official of the Meiji who had been friends with Kenshin as Battousai, to complete one last assassination task for him : The death of Makoto Shishio.
The Kyoto Arc
Aoshi, in the two year time skip between the Kyoto Arc and the base, had spent all of his time training in the mountains to become stronger than Kenshin, still trying to honor his friends. After feeling that he had truly mastered enough of his kodachi nitou ryu (that is, Double Kodachi style), he walked slowly and confidently to the Kamiya Kasshin ryu dojo, with intent of finally murdering the one known as the Bakumatsu's strongest. Upon returning to the dojo, the first one he saw that resided there was Megumi Takani.
He walked up to her slowly and asked her where Himura was, with her only reply being blatant silence. He looked her in the eye and stroked her cheek gently, and then told her that if she did not tell him where he was that he would kill her. She told him that she would never tell him where "Ken-san" was, in an act of defiance to the man who had once had control of her entire life.
Fujita Gorou, aka Saito Hajime
Saito Hajime (Kenshin's old rival) appeared the tell him that Kenshin had already left for Kyoto some time ago to defeat Makoto Shishio, the shadow of the Hitokiri Battousai that he had wanted to face for the past year and a half on behalf of his dead comrades. He asked Saito who he was, only to have the man reply with an alias that he had been using during his time being there, saying that he was "Fujita Gorou", a local police man sent to be their body guards for the time being. Aoshi left and told them that he would wait for battousai until his return. Saito looked on at Megumi as she screamed and lambasted him for his actions, asking him how he could dare to tell "that man" where Kenshin had gone, as he would surely kill him. He looked at her with a sly smirk on his face as he lit a cigarette saying that the man certainly had determination, however, with the weapon he was using, it may not have been misplaced.
Aoshi returned to the graves of his friends to find some unexpected visitors, four large men sitting atop their memorial stones and eating food, claiming to be Shishio's "Four Monks". They told him that they had been waiting for them, and that their master wanted him to meet with him personally, and to have him work directly under him with his Juppongattana (literally "Ten Swords").
Aoshi's reply was that if they did not leave this place immediately he would kill them and their master, and that he did not care for any of them at all, nor what their motives were. They said that they only got to see Shishio once a year, and that the offer being made to Aoshi was a privilege of the highest honor. As they said this, they told him that threatening them was not only a mistake, but also the fact that he would say something like that about their master meant that he was to receive a painful death at their hands. They said that not a single man had survived their "Four In One" Strike, and all leapt into the air with their weapons after spitting on the graves of Aoshi's friends, only to be ripped straight through by Aoshi's new Kodachi Nitou Ryu technique with great ease.
The Kodachi Nitou Ryu
Shortly after the attacks, a young boy by the name of Soujiro Seita, one of Shishio Makoto's most faithful servants, revealed his hiding place from the forest area. He told Aoshi that he had never seen anyone defeat their four in one technique so easily before, and that Shishio would really like to have a word with him. Aoshi said that he would never serve someone who would send out their own men only to gauge an opponents strength.
Soujiro patted his head with a slight drop of sweat on his head as he acted as though it was a simple act and normal for one to do, proud of Aoshi for noticing. He then said that Aoshi must be a fairly cruel man also if he knew that they were only pawns in the game and he still killed them anyways. Aoshi remained silent, and, eventually, decided to work for Shishio. During his time as Shishio's henchmen, he went to Kyoto on his own to look for Kenshin Himura, the man who had promised him a fight years ago, only to find Okina, the acting Okashira of the Oniwaban-shu, and defeated him in combat after revealing that he was set on his murderous intent. This was the first time that Aoshi revealed his techniques with the Kodachi Nitou Ryu style, and his extreme speed in front of the elderly man. He struck him down with technique, the Kaiten Kenbu Rokuren, a seemingly lethal move striking the base of the neck, but it seems he let him live. Okina commented later that there might have still been something left of the former man Aoshi was, sub consciously stopping him from killing his former sensei.
Aoshi is the calm, silent character of the Rurouni Kenshin series, who never seems to show much emotion in anything he does, except for slight anger in some portions of the series. He wears his traditional long white coat, and fights with kodachi blades throughout the series. When he reappears in the series during the Kyoto Arc, he seems to have a long sword, but in reality their were two kodachi left in one sheath, giving it the impression of a longer weapon than he really had.