The Phantoms of their Second Battlefield
It's Jyou who manifests his powers first, Taichi who drags them all into the war against the phantoms, Takeru who brings their partners back to them and Ken who gives him the final puzzle pieces with his unique powers, but Koushiro who manages to put it all together.
He wonders, at first, why his ability is lightning and a knack of finding whatever piece of information he needs for information or books. Why he doesn't have any power over data itself. Why, later, it is Ichijouji Ken who can manipulate both data and people's emotions (even if, at the beginning, accidental bursts of power led to him retching in the boys' bathrooms). But only when he's able to wipe phantoms off the map simply be dismantling their data that Koushiro understands.
And he understands why it was never easier, as well.
Digimon are made of data. Data, and also emotion. Phantoms are made of emotions, too, but they'd thought that was it. Until summoners like Takeru came along, summoning phantoms of their own from words or images or data.
And data is relatively straightforward to delete. When Koushiro is hacking the national database more than once, Ken is covering his tracks flawlessly for him. When the phantoms have been out for a while, gaining experiences and emotions, then it's harder. And Ken has nothing like Koushiro's thunder to protect himself.
At least Daisuke's never far off, or the others. They've got quite clear strengths and weaknesses, but their teamwork trumps all. Still, it's not until Ken polishes off a newly sprouted phantom that Koushiro realises it, that phantoms are like digimon and that's not just a convenient like they've told when Takeru brought their partners back to them.
It doesn't matter, though. They made the choice to fight a long time ago. If there'd ever been a choice at all.
He can't say he regrets it though. It's a realisation, but not one that will change them, or the world.