Snake Sannin
After I was effectively chased out of the hidden leaf I was furious. How could I not be? Over the course of two great wars and many more battles I had bled and killed for the village loyally and I was passed up for Hokage by a brat I had watched grow up. Then they dared accuse me of being a traitor and scum for using the dregs of the village in my experiments. My sensei was easily the worst though as he didn't even have the heart to kill me himself and let me go, he'd come to regret that.-
Thankfully I had the foresight to prepare in case something like this happened and had a base squirreled away in the land of rice. Annoyingly my other bases had been systematically exposed by that fool Jiraiya who was insistent on capturing me. The loss of them stung but was something I could accept as money was easy to acquire for me. A few years went by and my experiments had allowed me to discover a way to extend my life indefinitely. I had taken inspiration from the forbidden jutsu [Impure world reincarnation] to create a way for me to take possession of someone elses body by using my soul.-
There was a slight downside in the form of myself now being uniquely vulnerable to genjutsu but I had a plan to rectify this problem as well. Originally I was going to kidnap some random Uchiha to possess but after the massacre they were in short supply. Still though both of the options left were tantalizingly talented and one even had the legendary Mangekyou sharingan, the same eyes supposedly as Madara himself! I even subordinated myself to the rogue organization Akatsuki in an attempt to possess Itachi but he overpowered me easily with his powerful genjutsu.-
With that option effectively cut off from me I instead turned my attention back to Konoha were the last Uchiha in the village remained, Sasuke. That was not the only child in that village that interested me however as there was another that would have been my perfect vessel due to his talent in any other circumstance, Kenzo Mizuyama. The boy was a prodigy easily on par with myself or even greater from what my subordinate in the leaf told me. A strange unexplainable command over animals, a supremely durable body with extraordinary strength and flexibility.-
Godly chakra control and intelligence, it was all the things I could have possibly asked for in a host with the single exception that the boys body didn't have a whiff of any bloodline at all. All of these gifts were there innately without any explainable reason and as a scientist this bothered me greatly. Oh how I wanted nothing more than to snatch up the boy and disected him to learn all of his bodies secrets. That would be wasteful however and I was many things but that was not one of them.-
Instead I left the child to his own devices until I could discover a way to fuse a bloodline into his body safely. I had samples of a vast variety of bloodlines that I had collected during these years that I wanted to create the perfect combination of to create a vessel truly able to achieve my ambition of mastering every jutsu and living forever. Uchiha Sasuke would be a temporary vessel until I succeeded in transforming Kenzo Mizuyamas body into the perfect vessel. The sharingan were peerless dojutsu without a doubt but if I succeeded in fusing the bloodlines I had chosen and merging the result into the boys body I could have it and so much more.-
Time has passed since I left the Akatsuki and set my sights on the youth of Konoha and I had a few things to note during this period. First was that I had heavily underestimated the difficulty of fusing bloodlines together much less merging them into another's body successfully. I had been attempting to fuse the genes of the dojutsu clans of the world that I had samples from together with mixed results despite their shared ancestry.-
Yes that had been quite the surprise for me to accidentally discover, somewhere in the far flung past each and every clan with a dojutsu had a single ancestor that gave them their visual prowess. The fragments of that ancestors genetics were far too hazy after all this time for me to sequence unfortunately but it gave me a point that I could use to initiate fusion between each of these branches of this distantly related family tree. The Uchiha, Chinoike and Hyuga clans genetics unfortunately all shared a trait, they were all dominant.-
If one were recessive i could probably use that to fuse them together slowly but as they were all dominant any attempt at fusion tended to result in genetic clashes that served no purpose beyond crippling or killing the host I injected with them. Even when I tried to cultivate a genetic clone the result was always malformed , typically around the eyes i noted, or unviable. Fusing a single bloodline into a non-bloodline host was possible however as I had even successfully awakened a sharingan in one of my subjects who underwent the experiment. Though It should be noted that the eye was blue rather than red when activated due to a mutation in the process.-
My attempts to fuse the Chinoike and Hyuga bloodlines into hosts yielded similar results in the cases that succeeded. For the Chinoike clans Ketsuryugan the normally red eyes with purple horizontal goat like pupils were instead blue with red pupils. The Byakugan of the Hyuga clan were instead black as if the pupiless eyes had suddenly become nothing but pupil with no iris at all. The prowess each of these proto-dojutsu was less impressive than the normal thing however which was unacceptable and as such I refused to make more such shinobi.-
I was still determined to fuse these dojutsu bloodlines together but I had that relegated to a side project since it was clear it would take some time to make progress. Moving on I found that I had also underestimated the boys talent as my sensei had placed the child under my old teammate to learn, Oh did he learn. My spies reported extraordinary growth in the boy as he rapidly mastered iryonin jutsu and refined his own self created taijutsu under my old teammate. With little more than a year the boy even managed to create a solid clone technique that had me salivating.-
I took great pleasure in knowing that the boy refused to simply hand it over to my sensei. The boy instead merely wrote down the requirements to learn the technique and the risks involved in attempting to. I admit that even I was leery of learning the technique after reading what was written from one of my spies in the village. The price for failure was high, VERY high. The technique also had clear flaws that greatly lessened my desire to learn it as splitting my focus in such a way was likely to ruin much work rather than help.