You're Giving Me Too Much
If the Sage had not messed with the wheel of souls, he would not have remembered.
But to ensure that his son's were reborn of equal standing to enemy clans in the same generation, the Sage dragged Ashura through the wheel into the Senju clan's head family, and ensured that Indra's older brother would die.
For Ashura(now Hashirama)'s youngest brothers this was no problem. They were new souls, untainted by memories of past lives, but the second brother was not so blessed.
Thousands of years of memories stretched back in a mind with near perfect recall, crammed into a brain not yet equipped to process them.
In the first and last true stroke of good fortune in the newly named Tobirama's life, everything starts off so jumbled up in his head that it takes two years to sort things out enough both in his head and in the world around him to speak coherently. In that time the words he had been speaking (all garbled together because of all the different languages in his mind) had led his family to the conclusion that he was mentally deficient, good only as canon fodder in a war that has gone on so long that no one knows how it started.
To their credit they did not neglect him when the months of childhood development that he should begin to understand words and speak them passed with no sign of coherent vocalization, but rather than spending any more time trying to get him to read or write or speak they began to focus on his physical abilities.
It grounded him. Being able to focus on the physical world, the here and now of his body, centering him in his current reality. From there the memories sort themselves, separating like oil and water, settling into their places like silt settles to the bottom of a lake until it's disturbed.
He is Tobirama of the Senju; a clan of shinobi known for its thousand skills, each member unique, their natures individual and mastery of their crafts encouraged. He himself has an affinity for water natured chakra that is unparalleled and a natural sensory range that surpasses all that have been previously recorded.
But-
-he was once the archivist of the greatest magical library ever built;
-he was once a pioneer of bioengineering that invented the means to not only live sustainably in spaceships, but also terraform other planets with the same system;
-he was once a king of the fae court who felt the turn of the seasons as a human feels the rhythms of their own heartbeat;
-he was once a boy who lost everything and became a cyborg for the sake of vengeance;
-he was once so many disparate people across so many disparate lifetimes.
Only one thing remains consistent across lifetimes: the hunger for knowledge.
In every lifetime Tobirama has lived he has felt the burning desire for knowledge. How the world works, how to improve something, or everything, how to overcome the impossible. Always he seeks after knowledge, and in most lives he is able to pursue it, but sometimes he is derailed, by catastrophe, by sabotage, by priorities that don’t afford him the opportunity to learn everything he’d like, by circumstances beyond his control.
And now he has new things to explore, and has seemingly ended up in the best place to learn them all in. A clan reputed to have mastery of ‘a thousand skills’.