It began in the Academy. Both were gifted. Brilliant and powerful. Swordsmanship, flash step, demon arts. They came as naturally to the two prodigies as breathing. As with any pair of boys, being at a similar age and a similar level of skill, and each dividing the attention the other received, there were only two things that could happen. A rivalry, or a friendship.
Or, rarely, both.
Their similarities were remarkable, to anyone who took the time to look closely. Both orphans of lesser nobles, taken in by a great noble house. One by the Shihoin Clan, and one by the Shiba Clan. Both years ahead of their peers in terms of intelligence, skill, and strength. And both filled with a scientific curiosity that drove them to investigate and question things that no one else bothered to think twice of.
Such things as the very nature of their world. The forces that governed it. What effects those forces had. It was while contemplating such things that they found it. The deepest problem of their world. The thing they would call, for centuries onward, the last variable.
On a board, in a room, in a basement, of a building, in the Shinigami Academy, two boys unraveled a system of equations that governed the balance of the universe, and discovered that there was a problem. Something was wrong, and the numbers said that their would was doomed to fall out of balance. The system was unstable, and would eventually collapse.
They traced back through the history books and census records, compiling data and modifying portions of the equations. They pushed and pushed until the root of the problem was finally uncovered. A secret that would mean either the destruction of all things, or overturning the way that every soul viewed the world.
They began to work out a strategy to ensure the latter option.