The Doctor and the Master, coeval, contemporaries, growing up at the same time on Gallifrey centuries ago. There’d been other children, but they had been especially close.
It had been a relief when they’d both been accepted into the Time Lord Academy. The elation of both being chosen had made them giddy. They’d had such plans; they knew they’d always be friends.
But then had come the Untempered Schism, and while the one who would become the Doctor had been filled with wonder, the other had been driven insane.
Now friends had become enemies, still close, but forever in opposition.