Rin's an emperor of the enemy castle, while you're just a pharmacist by the other castle.

You both never really met face to face but you met thousands of times by medicines.

He's often fulfill his duties by joining wars and having his servants gets herbal medicines from you.

Till today, you were visiting his kingdom with your kingdom's Prince which is his enemy to make a bargain. And the ending decision he'd win and he wanted you to stay at his kingdom.