Innocence of a Child
In the anime, Soujiro's life as a child is seen in episode 55 entitled "The Tragedy of a Stormy Night".
Soujiro was born from an adulterous act of an important rice dealer. When his father died, his family only kept him "for the sake of appearance" which betrays that he was an illegitimate child. Instead of treating him like family and giving him a part of the family's business like the father intended, Soujiro was made to be their slave. He was forced to do labor such as carrying hundreds of rice sacks around warehouses for his step-family's business. He would be beaten by his drunk brothers for things that he did not do, laughed at by his sisters, mocked by his step-mother. As punishment for "his" mistakes, he would often be forced to sleep outside.
That Fateful Night
One night, as Soujiro was cleaning up his wounds, he heard a scream. Running to investigate, he becomes witness to Shishio Makoto's murder of three policemen. Shishio, seeing Soujiro, moves to kill him but instead spares his life in exchange for some bandages, food and shelter. Why? Because even in the face of death, he was smiling. When Shishio asked him why he was smiling, Soujiro at first did not realize what he meant. This was because it had developed into a habit that sometimes, he himself would not realize that he was smiling. It was his defense mechanism, that whenever his family would beat him, he would simply appear to smile the pain away.
Shishio taught Soujiro the motto, "The strong shall live, and the weak shall die.". As a gift, Shishio gave young Soujiro a wakizashi. This was hidden under one of the buildings, untouched until later that night. Meanwhile, Soujiro's family (after noticing all the stored bandages in the house were gone) realizes that Soujiro was housing the government fugitive. Running out, drunk, the family calls out to Soujiro with the intention of getting him to tell where the fugitive was hidden to claim a prize from the government. However, when Soujiro appears, their intention turns to murder.
Soujiro runs in terror, screaming for anyone to help him. He goes under the building where he had hidden Shishio's gift - the wakizashi. One of his brothers follows him under, and upon seeing Soujiro clutching the sword, foolishly unsheaths it. The rest of the family is beside the building where Soujiro was hiding. Hearing a scream, they assume that Soujiro had been killed by the brother who had gone under. A decapitated head appears from under. In a flash of terror, they realize that it was their brother. Soujiro climbs out from behind, smiling. Moving to slaughter the boy, Soujiro lifts his head, and in the thunder and lightning, kills them all.
It begins to rain. Shishio comes out and asks the boy if he was crying. Looking up at the sky, Soujiro smiles and says no. Shishio sees that his fake smile, was once again on his face. As Shishio moves to leave, Soujiro accompanies him. When Soujiro asks if he could be strong, Shishio promises Soujiro that he will be the strongest, next to him.
The Years of Training
When Soujiro meets Shishio, kills his family, and leaves his home, he was only 8 years old. From the time that he follows Shishio and his motto, the boy has to face the dark life of the underworld. Although it is not shown, Soujiro must come to terms with the life of an assassin. One cannot imagine what horrors he may have faced under Shishio's command, and what experiences he had that could have led to what he became, 10 years after.
Wanderings and After
Very little of this part of Soujiro's life is seen. In the anime, after being defeated by Kenshin and imparting his final gift to Shishio (to whom he owes his very life), he is glimpsed in the mountains, eating and talking with an old lady. There he tells her that he is headed for a life of a wanderer, seeking his own truth, his own answers.