Past

sea sickness in the ocean of wickedness

Nearly all we know about Jin's past is bound to his life as a dojo student. He was put into the Mujuushin dojo when he was very young, having lost his parents at an early age. He was closed off even as a young boy, devoting all his time and energy to kenjutsu; evidently, he had no friends. His frightening skill, coupled with his indifference towards injuring sparring partners, drove away the other children training at the dojo. His sensei, Mariya Enshirou, cautioned Jin that his strength was baseless because he fought only for himself.

Jin seemed to have a profound relationship with his master -- Mariya treasured Jin as his student, while Jin respected him as his teacher and possibly his benefactor. Possibly, Mariya took in the parentless Jin and gave some semblance of a father figure in his life. Mariya decided that Jin would be his successor as Mujuushin's leader.

At some point in time during his teenage years, Jin went to train with Niwa Juunosuke, a renowned martial arts master in Mihara. When Niwa-dono had nothing left to teach Jin, he made one last request -- if anything should happen to me, please take care of my sons. Jin seemed to be on some level of familiarity with the younger boys Kazunosuke and Tatsunoshin, as they addressed him as "Jin-nii" (nii = older brother) in Episode 18.

Jin also befriended Yukimaru, who was "like a younger brother".

Things suddenly changed when Mariya informed Jin that under Kariya Kagetoki's orders, the Mujuushin dojo would be turned into a school for assassins. Jin was indignant upon hearing this -- kenjutsu would no longer be an honorable practice, reduced to working for the shogunate. Mariya wasn't any more inclined towards the inevitable, but told Jin regretfully that it was the only way for the dojo to survive. It was a peaceful time; schools of kenjutsu were no longer as needed.

Apparently, Jin didn't back down on his disapproval for the decision, and resisted enough to prompt Kariya's fateful order -- Mariya must assassinate Jin. Mariya attacked Jin in his sleep, who instantly retaliated in self-defense. The darkness of night and Jin's lightning-quick reaction gave him no time to identify his attacker until it was too late. As Jin stared on -- absolutely stunned -- at the sight of his bloodied master leaning against the window, Mariya commented proudly on how much Jin improved. Those were his last words; he yanked Jin's katana out of his body, and died with the pool of blood slowly creeping towards his student...

Burdened with the enormous weight of this event, Jin hastily left the dojo with classmates in pursuit. Some, like Ogura, wanted revenge. Others, like Yukimaru, sought Jin's infamy. Jin himself didn't know what he was looking for -- maybe he was just running away, as he told Mugen and Fuu by the fire.

Jin faced down Kariya as Fuu hurried towards her destination. Although initially defeated, Jin managed to avoid being critically struck and returned -- just as Kariya was about to kill Fuu -- to finish his opponent off. He allowed Kariya to strike him, giving him the opening to finally avenge his master.

nobility?

It often is guessed that Jin is a descendant of the Takeda clan, owing to the four-rectangle kamon crest on his kimono. This would place him as a member of nobility, which would fit well with his dignified, standoffish personality. Maybe it's also how he came to acquire such a unique pair of glasses? (According to the pawn shop guy, they're worth a pretty penny.) Either way, Jin's ancestral origins are never touched upon in the series, so it's all conjecture as of now.