Hasegawa Ashitarō is a resident student of the Kamiya Dōjō in the 16th year of the Meiji period. Not much is known about his past except that he served Shishio Makoto and the Juppongatana before finding his way to the dōjō. He is the current master of Shishio's signature weapon Mugenjin.
Ashitarō has a very unkempt appearance compared to the other residents of the Kamiya dōjō. His brown hair is wild and ear length, his clothes appear dirty and ragged. His eyes are a brilliant purple hue, and he appears to have red tattoos around the base of his eyes. His base cloths consist a simple black gi and pants. He wears arm guards on both arms and sandals with foot wraps on his feet. Over this he wears a red and white robe in a similar fashion to his former master Shishio Makoto.
Ashitarō appears to have a fairly "no non-sense" personality. When prompted to lie about the prices listed on the brothel poster for Kubota Asahi, Ashitarō frustratingly proclaimed that it said whatever she claimed it did. This is due in part to Ashitarō being illiterate; claiming that reading "didn't put food in his belly".
Ashitarō is prone to getting into fights, and when doing so seems to lose himself in the moment. After starting a fight to defend Kubota Asahi at the Akabeko, Ashitarō ended up fighting with the police as well. It was commented the he forgot who he was fighting. This effect becomes even more apparent when Himura Kenshin intervenes to try and calm him down. This intervention has much the opposite effect causing Ashitarō to swing into a blind rage. In this mindset Ashitarō is able to effortlessly utilize techniques from the Three Secret Swords style created by Shishio Makoto, specifically First Secret Sword: Blazing Soul. After recovering from his outburst he appears remorseful, however this is short lived as he almost immediately gets into a minor argument with Alan.
Like other members of the dōjō, he is quick to complain when hungry.
- Himura Kenshin - Ashitarō appears to revere Kenshin to some degree. It is unknown exactly how much Ashitarō knows about Kenshin's past. While he does not refer to Kenshin as his current master, when Kenshin set the ultimatum that Ashitarō will not draw Mugenjin from its sheath again until instructed, he appears to comply.
- Kubota Asahi - Ashitarō appears to have some history with Asahi, mentioning that she once gave him food. She is taken aback that he remembers this small gesture. Later when Asahi is being gas-lighted by the owner of the brothel she is indebted to, Ashitarō sides with Asahi during the argument. This causes a fight to break out at the Akabeko.
- Inoue Alan - Ashitarō appears to have a very argumentative relationship with fellow dōjō resident Alan. Alan seems to often respond to Ashitarō with quips that cause the latter to comically lash out at him.
- Shishio Makoto - It is known that Ashitarō was part of Shishio's plot to conquer Japan, running errands for the Juppongatana. It can be assumed due to Ashitarō's possession of Mugenjin combined with is ability to perform its signature fire technique, Ashitarō may have either received training from Shishio or at least have seen the man fight.
- Akabeko Staff - One of the servers at the Akabeko comments that she found Ashitarō collapsed at their back door at one point. She appears to continue taking pity on the boy even after he moves into the dōjō, offering to allow him to work as a greeter and order taker at the establishment in exchange for a meal. (The status of this good will is unknown after Ashitarō started a fight with the police on the Akabeko's roof)
Ashitarō is shown to be a competent swordsman, being able to fight off several members of the police force without needing to resort to drawing his blade. When engaging with Kenshin, Ashitarō becomes enraged and releases Mugenjin from is sheath, igniting it and causing the boken that Kenshin is using in place of Sakabatō to catch fire and become useless.
He is noted several times to lack the ability to read and write, and when pressed claims that he doesn't need those skill. It is noted that his comprehension of written language is roughly the same as Kenji (who is about 4-5 years old)
Ashitarō was born in the town of Hasegawa in the Niigata Prefecture circa 1867~1868. He was orphaned at an early age and soon became a wild child living in the mountains, where he survived by stealing food, either farm produce or wild game, and was given the name Ashitarō (惡太郎), meaning "Bad Boy", by the local people.
Around 1878, Ashitarō stumbled into the Shrine of the Six Torii at Mt. Hiei in Kyoto, which was a secret hideout for a syndicate intending to overthrow the Meiji Government and conquer Japan. He was caught devouring food served for its leader, Shishio Makoto, who was impressed by the boy's skills to infiltrate his hideout and steal his food, so he decided to recruit the 'thief' as an errand boy for his syndicate, promising him that if he trained hard enough, he might have the potential of joining Shishio's Juppongatana(10 Swords) as his '11th Sword'. During his short time there, Ashitarō became familiar with fellow errand person Kubota Asahi because she once gave him food.
After Shishio's defeat, Ashitarō came into posession of Mugenjin, Shishio's last sword, which the boy discovered while fleeing the collapsing fortress. The sword was found missing a Tsuba and its Ito has become ragged (mimicking Ashitarō's own appearance). Due to the fact that Mugenjin was burned while in Shishio's possession at the end of his fight with Himura Kenshin, and then was buried in the collapse of the mountain stronghold; it is likely that the blade has been reset into an entirely new Tsuka.
After wandering for a while, Ashitarō came to Tokyo, where he buried the sword at a hidden shrine. He then spent some time at a run-down village known as Rakuninmura, and at one point was found collapsed on the back step of the Akabeko. Not long afterwards, Ashitarō was arrested for seemingly walking out on the bill for a meal, but it is implied that the authorities were aware of his involvement with Shishio's syndicate. Aware that speaking Shishio's name was taboo for the Meiji government, Ashitarō was careful not to mention it, so instead of being imprisoned in Hokkaido for the rest of his life, he was sent to a Tokyo jail to stay for the next five years.