Nickname: Ice Blue Eyes
Seiyuu: Yoshito Yasuhara
Birthday: January 1853
Height: 182 cm
Weight: 72 kg
Blood Type: A
Occupation: Okashira (leader) of the Oniwabanshu
Weapon: 2 Kodachi (short swords; Relative Size=11 to 15 inches).
Style of Fighting: Kodachi Nitou-Ryu
Nobuhiro Watsuki based Aoshi on Hijikata Toshizō, the vice-commander of the Shinsengumi. There are most certainly versions of Hijikata portrayed in books and fiction; Aoshi grew out of the Hijikata who killed his gentler feelings and buried his human weakness. Watsuki describes himself as a fan of the other version of Hijikata. Watsuki describes that version of Hijikata, seen in Moeyo Ken (Burn, O Sword), as "a bundle of raw combat-instinct who keeps fighting until the very death." Since, according to Watsuki, the addition of the Oniwabanshū occurred during the "last minute," he found difficulty writing with him since he had not resolved a "concrete image" for Aoshi. Watsuki says that he used no specific design model for Aoshi. As the image of Hijikata grew stronger within Watsuki, the Rurouni Kenshin author added fringes (bangs) to Aoshi's design. Watsuki held a chance to change the hairstyle while compiling the edited manga, but chose not to edit the hairstyle, since he did not want readers to believe that "Aoshi was wearing a rug or anything." 
Watsuki said he originally intended for his design of "young Aoshi" to be used for another character. He says that many female readers liked young Aoshi. He described drawing Aoshi's fringes as "a pain."
Shinomori Aoshi was raised a ninja of the Oniwabanshou, who worked for the Shogunate government during the Edo period. He helped to raise Makimachi Misao from childhood as a member of the Oniwabanshou. At the suggestion of Kashiwazaki Nenji (better known as Okina), Shinomori was given the position of Okashira at the age of fifteen, in time for the Oniwabanshou to defend Edo Castle.
After the revolution, since a few members of the Oniwabanshou were unable to adapt to life in the Meiji era, Aoshi and these members worked for the corrupt, power-hungry business man, Takeda Kanryū instead. Aoshi's Subordinates included Beshimi, who specialized in darts and poison, Hyotoko, whose name literally means "Fire Man" and uses flint-and-steel teeth to breathe fire. Also were Han'nya, a skilled martial artist and deft ninjutsu practitioner (also his unofficial second in command), and Shikijo, a "muscle man".
Shinomori Aoshi is a very quiet, intelligent individual. He never speaks when unnecessary and he always thinks before he acts. He seems to be unemotional most of the time, earning him the nickname of "that gloomy jerk" from Myōjin Yahiko. Aoshi only smiles once in the manga (excluding the weird smirk when he sees Kenshin cut a lamp post in two) and around eight times in the anime. He doesn't show it, but he has a soft spot reserved for his friends and Makimachi Misao.
Aoshi appears to be very cool and level headed when we first see him, although we see glimpses of his obsession to defeat Kenshin in a few scenes. After Hannya, Shikijō, Beshimi, and Hyottoko die, and during the time Aoshi escapes from Kanryū's mansion, he seems to be in a shock, and his eyes have a shaded look, a conventional manga portrayal of depression, possession, or daydreaming. His intelligence and analytical skills are also well-honed evidenced from his display of narrowing the location of a mine below the water to a tenth of a degree and meter.
Aoshi drawing his double kodachi.
Aoshi is a grand master and Okashira of the Oniwanbanshou shinobi. Aoshi's weapon of choice is a kodachi, a sword that is described in the series to act like a shield because its light weight makes it easy to block with. He originally used only one of these short swords for defense outside of Kaiten Kenbu and relied mainly on kenpo for his offense, but later used a two-sworded style, employing a pair of the swords. When in their saya, they appear to be part of a single sheathed sword, one blade appearing to be the hilt, while the second blade is stationed opposite the first and blends in with the saya itself, giving it the impression of being a nodachi. A dangerous opponent, he uses his speed together with a mix of sword and hand-to-hand combat moves. His physical strength is extremely high, having once knocked Sanosuke out with on blow to the back of the neck (in the anime).
* Onmyō Kōsa: (Shadow-Light Cross) Aoshi blocks an enemies striking weapon with one kodachi, then strikes it with the second kodachi, thrusting the first one through.
* Jissen Kenbu: (Actual-Battle Sword-Dance) Combo attack using "Ryūsui no Ugoki" to flow around the enemy, confusing them with a constant change in speed. Aoshi will then strike from various points, slashing them with his kodachi, then ending it with "Kaiten Kenbu". Before deciding to use a two kodachi style of fighting, this was Aoshi's final attack. Though he is nearly impossible to hit during the Jissen Kenbu, he is vulnerable when he switches from a defensive to an offensive stance, a weakness exploited by Kenshin, and later Shishio.
* Kaiten Kenbu: ("Spiral Sword Dance" or "The Dance Of The Wheeling Sword") Aoshi claims to have used this attack to defeat "all who have intruded upon Edo castle". It involves Aoshi rapidly spinning around three times, while holding the kodachi backhand, to slash the opponent thrice in an instant. Aoshi usually uses this attack after disorienting the opponent with Ryūsui no Ugoki.
* Kaiten Kenbu Rokuren: ("Spinning-Heavenly Sword-Dance Six-Series" or "The Dance Of The Wheeling Sword Six Successions") This attack is Aoshi's ōugi. The attack involves Aoshi gripping his kodachi backhand, then slashing the opponent six times in rapid succession, from both sides (it relies on confusing the opponent on the direction from which each subsequent strike is going to come). It's basically Kaiten Kenbu but done with two kodachi.
* Onmyō Hasshi: (Dusk to Dawn Strike) Aoshi throws both kodachi at the opponent, hitting the end of the first one with the second one for an extra boost. The second kodachi is hidden directly behind the first, so the opponent would only see one kodachi heading for them in their line of sight.
* Gokō Jūji: (Yin-Yang Cross) Aoshi crosses the two kodachi and moves them outwards in a scissor-like cut.
* Ryūsui no Ugoki: (Flowing-Water Movement) This is moving technique where Aoshi moves around silently, rapidly alternating between fast and slow movements and confusing the opponent due to the fluid motion & continuously changing speed. It can be beaten by watching for the moment the user attempts to strike.
While working with Kanryū as "muscle" in his opium manufacturing operation, Aoshi came face to face with Kenshin, who was trying to rescue Takani Megumi. During the confrontation with Kenshin, Aoshi revealed that he aims to kill Kenshin, who was formerly known as the famed Hitokiri Battōsai. After an intense fight, their battle ends with Aoshi's defeat when Aoshi is struck a second time to the throat by his own Kodachi after Kenshin narrowly escapes death by having his scabbard reduce the damage brought on by Aoshi's Kaiten Kenbu. Kanryū betrays Aoshi and kills his four subordinates with a Gatling gun while they die one after the other protecting Aoshi, and when Kenshin had gripped his sword Kanryu ran out of bullets (in the anime, Beshimi through a dart into the ammo belt, which jammed, allowing Kenshin to get in close) and was soon struck by Kenshin's sakabato and arrested. After the deaths of his friends and team-mates, Aoshi became even more determined to defeat Kenshin, obsessed even, stating that it is the only thing he can do for his fallen comrades now, to obtain the title of the strongest and bring it before their graves as flower.
Determined to achieve his goal, Aoshi follows him to Kyoto after training intensely for a few months and narrowly missing him when Kenshin heads to Kyoto with Saito and Misao, and joins Shishio Makoto (despite initially rejecting the offer) in hopes of fighting Kenshin. He lets no one stand in his way, going as far as fighting and severely injuring Okina (Kenshin however, later remarks that Aoshi subconsciously held back against his former comrade) to try and learn of Kenshin's whereabouts (but fails to get the location).
Towards the end of the Kyoto Arc, the much-anticipated battle arrives and Kenshin fights Aoshi. Kenshin saves Aoshi from his own madness, as he realizes that death is not the answer to his situation. To put an end to the feud and see who is truly the strongest, both decide to use their strongest attacks resulting in Kenshin's victory. Even though Kenshin's ougi, Amakakeru Ryu no Hirameki, activated after Aoshi's Kaiten Kenbu Rokuren (which was already cutting his neck), the sheer speed of the attack allowed it to strike first and incapacitate Aoshi. He later appears to fight against Shishio, and although he proves no match for him, he managed to buy time for Kenshin as he struggles to get up after Shishio knocks him out with the second secret sword.
Aoshi spends his time meditating in a temple, needing time to sort through what he's done and been through. Before Kenshin leaves Kyoto, he asks if Aoshi would like to share a drink (of sake) with him; Aoshi refuses the offer (saying that he does not drink), but goes on to say he would be willing to have tea with him when they meet again.
In the Jinchū Arc, Aoshi returns to Tokyo with Misao in order to deliver the diary of Yukishiro Tomoe to Kamiya Kaoru. They arrive to discover Kaoru is "dead," though Aoshi soon solves the mystery and reveals that the body buried of Kaoru was actually a doll and defeats the puppet master, Gein, who created the doll in the first place. Subsequently, he defeats Gein in the graveyard after the latter returns to try and retrieve his creation which he considers his masterpiece, and allows the latter to burn in the flames he himself started after telling him that Aoshi burned the doll as a sign of pity. Joining Kenshin and his friends as they travel to Yukishiro Enishi's island to rescue Kaoru. Aoshi defeats Suzaku, a master mimic who is one of the four Sushin bodyguards of Enishi's second in command before leaving Kenshin to fight Enishi. Following the battle, he and Misao return with the group to Tokyo, only for Aoshi to announce that he and Misao will return to Kyoto very soon because the high mountains where Aoshi buried the remains of his four minions will hit winter before the plains do, and they'll have to give them a finer resting place. Before leaving, he honors Kenshin's request in Kyoto by having a cup of tea with him.
Informations sources: Wikipedia & Let it burn