Tobirama faces a dilemma each and every time the Senju battle against the Uchiha. He knows better than to hope that his soulmate isn’t a shinobi—and to hope that they’re good enough to evade his blade. He’s somewhat, begrudgingly, thankful that Izuna seems to be his equal on the battlefield and spare him the lingering doubts each time he goes for a killing strike.
(He really doesn’t want to care for his soulmate. But even he’s not immune to all the lovey-dovey stories that get shared around the compound. It’s illogical and makes absolutely no sense—but to think, to believe, that there’s a person who would understand him, even better than his own anija? Than Touka? His traitorous heart skips at the thought.)
So during the few hours of sleep he gets that doesn’t overlap with his soulmate’s, he watches. Because after the first battle with the Uchiha and his subsequent panic that he may have cut down his soulmate (and wouldn’t that be his luck) he supposes he should at least figure out who they are, so he can make sure they stay alive. Not that he could ever be with them, not with Father still as clan head. But the thought that they live is a pleasing one.
Tobirama learns that his soulmate starts his days with an almost meditative stretching. There’s only brief glances as they’re stretching, since apparently they like to keep their eyes mostly closed—but they do the stretches outside, in the very early morning light.
The first piece of identifying material he sees—asides from that they are Uchiha—is all he needs to identify his soulmate. In particular, he sees a familiar face that he never, ever wishes to see up close: Madara. And by the way his soulmate is so obviously clinging to him, it must be a close relative.
Dread pools and freezes in his chest, because that would mean—
Well, Tobirama thinks wryly, at least he doesn’t have to worry about accidentally killing his soulmate if he’s always the one battling him.