Abayama was a good friend of Anotsu Kagehisa's father, and the former has known him for the whole of his young life. When Antosu's grand father first showed Anotsu, training with Sabato Kuroi to Sosuke, he told him he was going to kill him, as Anotsu planned to take over the Itto-ryu. The problem was, he was no longer strong enough, and wanted Sosuke to either distract Kuroi, or help him. Sosuke told Saburou that he would help him make the Itto-ryu better. Saburou thought that meant he would kill Anotsu, but later Sosuke kills Saburou, and leaves his body for Eiku Shizuma, Anotsu kagehisa, Taito Magatsu, and Sabato Kuroi to see. When Anotsu began forming the Itto-ryu and challenging schools, he approached Abayama to be the second-in command because of his tremendous fighting skills and knowledge in tactics and politics. At sometime Abayama left his wife and his then still young son alone, the reason behind this is unknown.
Sosuke is shown various times throughout the plot with his assistant Makoto, who was really a spy for the Mugai-ryu, but not many of them are noteable except when he notices that the forged letter Makoto gave him that called for one Itto-ryu member to come meet him, was not done by Anotsu's handwritting. Resulting in Sosuke sending three Itto-ryu instead.
Banquet of DeathEdit
When Sosuke Abayama called for a Banquet between a group of other Itto-ryu members, including Monk Boa, to boost Morale between the group, they enjoy themselves for a while. When they begin to realize that their food,and drinks had been poisoned, Kagimura Habaki, and Giichi appeared, and killed every single Itto-ryu member there except for Sosuke who barely survived with a cut from Habaki.
After it is found out that information was being leaked to the Mugai-ryu by someone from the inside, Makoto left. One day Abayama finds Makoto in town, and proceeds to chase him to a secluded area. An exhausted Makoto claims that he was just bringing Sosuke to a good place to kill him, thinking that Sosuke was unarmed. Sosuke pulls his dagger from his sleeve, and Makoto draws his katana, but is easily defeaten and killed in one attack.
When Abayama's son is tortured by Shozo Murasaki of the Rokki-dan, it is revealed that the Itto-ryu is still at the same sword school even though it was burnt down. The Rokki-dan find a trap door, and all but three of the Hebigumi there go underground. They see Sosuke, and all but one Hebigumi who reports to Habaki, follow sosuke deep underground to a large damp room. Sosuke goes behind a locked door, and Amon raises a huge gate leading to the ocean. The entire underground is flooded out, and all of the Hebigumi underground are killed, and the one reporting to Kagimura is shot up in the air.
Leaving EdoEditWhen the entire Itto-ryu is banned from Edo, Sosuke leads the new recruits, and Koji Kashin to Hitachi. Along the way Koji splits up, and ends up in order to get some of the Itto-ryu off their tail, but he is killed after defeating Inroku Ban the extremely skilled sharpshooter of the Rokki-dan, along with five hebigumi. Sosuke, and the others stay at a shack for a while, and are confronted by Shishiya Arashino, Shozo Murasaki, and Tarieshin Hasshu, who cut off Yatsushiba, of the Itto-ryu's head along the way. When they arrive they are told that if they wanted revenge they would need three trump cards to face the Rokki-dan, and that Anotsu's body was found along with three other Itto-ryu members. Sosuke calls Shishiya's bluff, and says that Anotsu's horses were probably just killed by a Rokki-dan member before he could get to them, and has just been delayed. He also says that if Anotsu was truly dead, the Rokki-dan would have no reason to attack the rest of the Itto-ryu because they only needed to kill Anotsu. The three Rokki-dan are breathtaken and begin to wonder if it was a waste coming, but Sosuke challenges them to a three on one battle. Every other Itto-ryu member quits, in order to break the rule of one on one fighting, and the three strongest of the new recruits Ariusu, Koda, and Yatoin carry Sosuke away to safety, while the other six fight the three Rokki-dan. Sosuke convinces Ariusu to stop, claiming that he is going to throw up. Sosuke tells the three that the new recruits wouldn't be able to win even if it was thirty on three, let alone six one three, and that the three should be there helping. Giichi, and Hyakurin appear before them, and Sosuke recognizes Giichi as the man with Kagimura at the Banquet of death. Sosuke breaks out of the wraps the three Itto-ryu put him in, and attacked Giichi, cutting his ear clean off. After a short fight Sosuke reveals his fake right arm, and the three weights he extends on it to make it almost like the end of multiple Kusarigama. Sosuke easily overpowers Giichi, and Hyakurin attempts to help him, but the three Itto-ryu members with Sosuke attack her one at a time. Hyakurin jumps up on the horse, and up to the graveyard, and when Koda does the same after dodging her arrows, she shoots the horse, which runs into the middle of the fight, and knocks Sosuke, and Giichi down. After more of Sosuke's domination over Giichi, Sosuke remarks that even though Giichi is getting his ass kicked, he hasn't even broken a sweat, when Sosuke himself is somewhat exhausted. Mitake, and Hyakurin come to help Giichi once again, after his Itto-ryu are killed, but Giichi tells them to stay out of it, and tries to attack from above, in order to make the chains lose effect, and not hurt him. Giichi lands, and stabs Sosuke in the chest, but as he gets back up, Giichi jumps on top of Sosuke, and proceeds to beat him. Mitake hands Giichi a Tanto, which he plunges into Sosuke's chest, and leaves him there to die.
Abayama Sosuke is easy going and friendly. He enjoys daily baths and the nice things in life, like tea and sweets. It seems he does not like to be old, as he complained about it to Makoto. When it comes to fighting he is a terrifying opponent, whom the strongest fighters have problems with, even though he is in his sixties.
Weapons and fighting skillEdit
Despite not looking like it, Abayama is still in a very good condition. He managed to follow a running Makoto whilst not showing any signs of exhausting, despite Makoto clearly being exhausted. He is quick and agile, and him lacking his right arm did not change anything in his fighting skills. He has two weapons, which he both conceals in the right sleeve of his robe. The first and his main weapon is a dagger/machete like weapon, which he can use for offense or defense. It is designed to be used with one hand. He can easily spin the weapon with his fingers to change from defense to offense. The second weapon is a pair of chains with weights attached to the stump of his right arm. He can use this two chains to attack and defend, similar to a kusarigama(without the sickle). He used them in his fight against Giichi and the Rokki Dan.