Ezo refers to what later generations call the Ainu people.
The Ainu were originally spread throughout the islands of Japan, real indigenous people, but not anymore, of course. They'd already been driven all the way from Kyushu Island into Ezo by the "Wajin," and the "Expedition Against the Barbarians" part of the top Japanese Samurai title—General of Expedition—was all about campaigning against the unlucky "barbarians," the Ainu.
Now, the Ainu have all retreated into this icy, snowy Ezo Island, but the Wajin aren't letting them off. They've already started building settlements and "trading posts" on Ezo, though since the Japanese aren't used to the snow and ice and tech is still so backward, all those villages and outposts are crammed into the southwest corner Matsushima region. No one's managed to push deeper inland yet.
