When Kyran turned 18, his life turned upside down. The Royal Army raided his clan because his supposedly non-existent magic got exposed. On that fateful night, he was the only one who escaped. Up to the present, the status of his clan was unknown. A magic inheritance that became a curse, Kyran became a fugitive. Always on the run... living in fear when the Royal Army finally captures him. At least that's what most people thought... "I'll definitely save my family. And I will make all those who made my clan suffer pay tenfold. I'll make use of this magic they so feared, and show them what real terror is...." Disclaimer: The cover image isn't mine. I'll replace it when I finished mine.
When Kyran and Dusk first met, Dusk left in the direction of Armakea. Thus, when Kyran warped to his location, Kyran knew he was inside Armakea's territory. If not, Kyran would not have succeeded in warping to where Dusk was due to the warp's target distance limitation.
Considering this, Kyran disguised himself as Achim when he left Dusk's hiding spot. He could have a number of people from Armakea he could change into, but going for Achim gave him a wider width to move around. After all, Achim was a Prince, and he could be arrogant, and no one would dare defy him given the King's favor. Moreover, Achim was his Servant.
Although Kyran's location was closer to the border, which could be considered under Kesiah's jurisdiction, Kyran could not change into her for obvious reasons.