The last person invited was Kupa, a Waterbending grandmaster from the Northern Water Tribe.
Kahn was less familiar with this grandmaster, as the Water Tribe had faded from public view long ago.
The Water Tribe was based at the two poles and divided into two major tribes, the Southern Water Tribe and the Northern Water Tribe. Many years ago, the Fire Nation had already wiped out the vast majority of the Southern Water Tribe's forces. Now, only a small remnant roamed the seas, harassing them and striking back, but to the Fire Nation, these attacks were no more than an itch.
Only after successfully conquering the South Pole had the Fire Nation discovered how desperately scarce its resources were and how harsh its environment was. There were simply no resources to be gained; one could even say it had no value whatsoever as occupied territory.
Thus, after expending considerable effort to conquer the South Pole, the Fire Nation had voluntarily abandoned it, while also abandoning its plans to attack the North Pole, whose environment was comparable.
The Northern Water Tribe was stronger overall than its southern counterpart. Coupled with the fact that the northern environment inherently favored the Water Tribe in battle, the Fire Nation truly had no reason to waste manpower and energy on a place with no use whatsoever.
The Northern Water Tribe seemingly had no intention of avenging its kin, either. Once the Fire Nation abandoned its plans to attack the North Pole, they were content to remain tucked away in their corner, living closed-off lives apart from worldly strife, gradually fading from public view.
Even so, Kahn attached considerable importance to this trip, not only for the valuable support this Waterbending grandmaster represented, but also for something he had once happened across in an ancient book.