Other than him killing all the students in the graduation ceremony the series didn't clearly mention anything else about Zabuza's early years. 'Well i guess, there really isn't much to it.. I am an orphan and also part of the lowest caste in the Hidden Mist Village.'
Although there was no mention of it in the series, but it seems that, Kirigakure's society was split into a caste system. The highest-ranking civilians were those whose family lineage originally stemmed from within Kirigakure.
The second caste was for members of families who were allied with Kirigakure on the battlefields in the past. While the last and lowest caste consisted of members of the families who were defeated on said battlefields and annexed into the village, Zabuza being one of them.
'I wanted to die in my last life. I got that wish, but not only that, I was even given a new chance at life, as Zabuza, so I guess it worked out either way.'
'Currently, the Third Mizukage is still alive, while the Third shinobi World War is still ongoing, so I have a basic idea of the time period that I am currently in. I have to grow my strength quickly before the Fourth Mizukage takes over.' Although, the village is already being called the Bloody Mist Village, it will become even bloodier during the reign of the Fourth Mizukage.
'Right now I am nine years old.... If I remember correctly Kakashi and I are of the same age. The war will probably last for at least another four years. Kakashi had to kill Rin at around the end of the war, after which Obito goes mad... So that means I only have four years to prepare myself.'
'I won't be able to reach the Fourth Mizukage Yagura Karatachi's level by the age of 13, but I still need to be strong enough to face the storms that obito will create on his path of revenge and world peace'
'But how?' Although he liked challenges, he certainly wasn't foolish enough to dive into the sea without any preparations. 'Only when I have greater strength will I be able to face greater challenges. But the question still remains..... how do I gain more strength??'
Zabuza didn't have any unique heritage. He wasn't from any special clan or had any kekkei genkai (Bloodline Limit).
'The only thing I have is a strong body and an above-average chakra reserve. Though the Zabuza in the show was shown to be stronger than an average jonin, he wasn't at the level of a prodigy like Kakashi.'
Even though Zabuza had trapped Kakashi with his water prison, one has to remember that after Kakashi was released, he ended the fight fairly quickly. Moreover even comparing Zabuza with Naruto part one's Kakashi is rather redundant.
During the period following Rin and Minato's passing until his encounter with Naruto... Kakashi experienced a period of stagnation in his training and personal growth. Depressed and Wrestling with grief, he lost a large part of his early youth.
'Looking at the things I can do right now, there isn't any special training I can do. So starting today, I should just work on my kenjutsu. while my long-term goal should be to get my hands on one of the seven great swords by becoming a member of the Seven Ninja Swordsmen of the Mist.' He gets out of the hospital bed while planning out his future.
Just when he was getting ready to leave the room, the doctor comes in. After a short checkup done by the doctor, no major complications were found in his body, leading him to be discharged from the hospital.
Walking through the relatively quiet main road of the village, he eventually reaches the outskirts. Along the way, most of the villagers chose to ignore him, even if they had recognized him.
It seems no one wants to associate with him... A sentiment he agrees upon... He, too, doesn't want to be bothered by other people. It is a win-win situation for everybody...
The area around him is a sparsely covered forest. The trees here looks like what you would expect from an island like Kirigakure. He could have gone to the village's training ground, but he chose not to. After staying in bed for nine days, he wanted to stretch his muscles alone.
He first does some hand and leg stretching, followed by twisting and turning his head and hips. repeating it for half an hour, he was done with the stretching. after that, he runs along the banks of the river that flows along the forest.
As hours passed while he ran, a rain of sweat continuously streamed down his brow..., drenching his flank jacket and cuff rolled trousers..., running barefoot, his feets are muddy, earlier he had taken off his shoes and gloves, the accumulation of all that sweat had made them sticky with an unbearable smell...
"Judging by the distance I've covered in a loop, I must've been running for more than three hours, and I didn't even use any chakra. Even though I'm only nine years old, my body is remarkably strong!"
"This much is enough for today." Deciding to stop his physical exercise, Zabuza walks out of the forest.
Rustle!... Rustling could be heard as a figure appeared from behind a tree trunk. "He finally decided to end it for today, huh?" A hidden sensory ninja on his lookout shift had noticed his activity. it seems that, he was keeping tabs on Zabuza's moves.
"Now should I keep this in record?"
Unaware of the little dilemma he had created, Zabuza walks out of the forest. even though he had enough chakra to use flash steps, Due to the exhaustion from running for hours, his body wasn't able perform them right now.
Exhausted, he directly goes home; a two room apartment, in one of the tall residential buildings of the village.
Early next morning... Knock! Knock!... Someone was knocking on the door. startled by the noise, Zabuza wakes up. He goes on to pick his kunai, but then again... decides against it. If someone strong really wanted to harm him, they wouldn't come from the front door and if they really did, a relatively strong Chunin like him wouldn't be able to do much.
Opening the door, he finds a letter held down by a shuriken, keeping it from being blown away by the wind. Picking up the letter, he proceeds to read it, the contents were simple, he is ordered to meet his new genin team, which he was supposed to be part of just after the graduation ceremony.
The meeting is supposed to take place at 6 o'clock. noticing the meeting time, in a hurry he searches for his pocket watch, finding it under his bed "Phew! It's still 5 o'clock. I still have time before the meeting, grumble!... guess i have to find something to eat first..., now that i think about it... i haven't eaten anything since i got out of the hospital!"
Living alone, everything from cooking to cleaning, he had to do it himself. Finishing his morning routine, and after stuffing his mouth with some bread and milk, he walks out of the building and unto the practice ground where he is supposed to meet his team.