Kenshin was born to the Kuroda family many years ago. Being the first born, he was expected to carry on the family name. His father, Arashi, was a renowned swordsman turned blacksmith. Although he no longer crafts instruments of death, the work of Arashi Kuroda can be seen all over his house.
Its actually fairly easy at a younger age to realize that you're the one expected to mimic what your father has done. Kenshin was put through rigorous training and tutoring. He received lessons from friends of the family, namely a former sensei to his father named Sengoku Kaien. Now, Mr. Kaien may appear to be a all-around nice guy, his training methods are brutal. Being born into the Shinigami world has its perks, such as healing techniques. When Kaien would injure young Kenshin in training, he would simply summon the family nurse to patch him up and resume training.
By doing this, he was able to gain the skills at a rapid pace, becoming stronger and smarter accompanied physical and emotional trauma. One time he got angry with his father and Kaien for their harsh training methods. At his outburst, the boy's father seemed to remember the training he had himself gone through years and years ago, and quickly made it clear that displays of anger, especially directed at one's father, were not tolerated his house.
And such was Kenshin's existence. He trained and learned daily under the harshest of methods, but only because his parents thought it would be ok for him to learn that way. So long as he kept his negative emotions in line, he would be left with less bruises too. He didn't really like this setup, but there wasn't really anything he could do about it, so he accepted it.
Even death gods must die...
Though he was familiar with death, growing up in Seiretei, Kenshin never had to deal with it personally until he was about ten. It was found out that his mother had been struck with a horrible disease. It was inoperable, and unable to be healed by any means as it attacked her entire body at once. The only thing that they could do was to sit by and watch as their mother wasted away.
Once she had finally passed, for a while something seemed off in the household to Arashi. He couldn't tell what it was at first, and then it struck him. His father. His father was missing from his usual position in the family. He wasn't blacksmithing or practicing his kendo. It was confusing to say the least. Though he wasn't happy with it, Kenshin had grown used to the sound of his father hammering away at burning hot steel or recapping their glory days with Kaien. This new, somber father just felt like a pale imitation. When he too passed, Kenshin wanted to mourn him, but didn't bring himself to do it. He could only see his old father, standing over him and telling him that swordsman showed no grief. As silly as it was, to Kenshin, his father's teaching finally gotten into him...
The academy....
Just like his friends, Kenshin found himself in the academy. He had trained along with them; so he saw no reason to let the skills he had been working on go to waste. While his friends had their own strengths and weaknesses, Kenshin seems to have been set on a path of becoming an specialist. Of course, he found certain things to be interesting as well. Hakuda was one such subject. Arashi was able to memorize and utilize attack patterns more easily than the others. This gave him some sense of body control that training in swordsmanship did not offer him.
Kido, on the other hand, tried its best to escape him it seemed. Try as he might, he couldn't manage to grasp the natural skill that some of his friends seemed to command. He wasn't the strongest in the world, but he would have a lot of work to do if he ever wanted to be as capable a spell caster as some of his friends.
Enter the dragon...
It was some time after he had graduated from the academy when he first heard his zanpakuto spirit. Though he had been decent in his zanpakuto resonance class, and had felt their presence, not once had he heard a word from them. It was a little maddening to say the least. At first, when he felt even the slightest squeak from his sword, he would rush into introducing himself and then it would be silent...
He finally heard him for the first time one day after a particularly bad mission. A foolish comrade had blown their cover and the group of hollows they had been following had attacked. It frightened him, but his father's words of not showing fear came back to him. The only thing he could think of was how Kaien would laugh at his misery and that only encouraged him and he charged at the group of hollows, seemingly alone...
When he awoke, he found himself lying on a cliff. The sounds of the breeze weaving through grass past him by. He looked around for a moment and was confused. What was he doing on a grassy hil, and how did he get there? He got up and brushed himself off and began wandering around. He couldn't tell just how long he spent wandering until he came to the clearing and saw the tree. Majestic and ancient. He had only five seconds to admire the beauty before he was attacked. He doubled over, trying to protect his head from the blows raining down on it from an unknown source. He finally managed to throw off his attacker and found it to be a large man, similar in age to himself. His calm demeanor after the sudden attack, his almost foolish and otherworldly attire and his sword tied to his waist confirm to Kenshin that the man he was looking at was his zanpakuto spirit, Hiryu.
At first they just stared at each other. Kenshin with confusion, and Hiryu with cold disdain. When he finally asked why he was so cold, all he said was that he was trying too hard. With that, he turned from Kenshin and went over to the tree atop the hill. He followed him and sat next ot him by the tree. He explained, nonchalantly, that the reason he had never spoken to him before is because he thought him to be too much of an overachiever. That he was not worth him protecting.
He had finally thought him worthy of audience with him when he charged the group of hollows. He had realized that it wasn't that he wasn't courageous or trying too hard, he was just trying to do what is right for his friends. He said his way of keeping this courage from showing was stupid, and that nothing good would come of it. Kenshin, now understanding what was going on, merely smiled and stared at the starry sky.
This, of course, earned him some animosity from his spirit, but instead of turning from him, he decided to teach him his release phrase. Perhaps saying the phrase would help him feel the fires of courage that should be shown sometimes. It didn't but the incident did start the relationship of zanpakuto and wielder to the levels it was destined to reach, though neither of them could be entirely sure just where that was.
To sleep, and to wake....
When Kaien fell into a coma, Kenshin was distressed to say the least. Although Kenshin felt a brief moment of glee, he felt guilty and never thought it again. He didn't want to lose his friend, his teacher, as easily as he had lost his parents. After all, he had looked up to Kaien growing up, and had been most fond of the few moments when they laughed together while during their lessons. They said one day he might wake up, but Kenshin, predictably, was impatient. He didn't want to have to wait for his friend to wake up. He made a deal one night by Kaien's bedside. If Kenshin got strong enough, Kaien would wake up. Perhaps a bit childish of a thing to make, but what was he supposed to do? The doctors had treated the situation like adults, but that didn't do anything.
Kenshin then left him while taking his pipe and herbs. He knew how precious these two items were to Kaien. He knew full well that if he were to wake, Kenshin would face the wrath of a thousand suns for committing such actions. He didn't care. He would keep them as memento and as motivation to get stronger. Not only because he looked up to Kaien, also because he wants to avoid his wrath as much as possible.
This was the story of Kuroda Kenshin before his ascension to become the captain of Squad Six. His old friend Sengoku Kaien did wake from his coma. However, he was stripped of all of his memories, and that included his fighting prowess. Now, Kaien wanders the world, seeking what he claims to be 'The Truth". The pouch and pipe that Kenshin took years ago have been returned to their rightful owner, in the hopes of sparking some sort of memory. It did, but not all was regained. Kaien remember the time he spent with both Arashi and Kenshin during Kenshin's formative years, but nothing before or after.