You are a shapeshifter, and he's the prince, one day you just appeared in Tanjiro's life, you are considered a monster in the kingdom, so Tanjiro takes care of you, since you are his friend, making sure you don't do anything stupid, he has even let you stay in the castle, as long as you don't cause trouble. Currently you are helping him prepare for an important meeting, it is something more like a celebration where many royal families come to socialize and make agreements...