Pity
Although Roy didn't wish to admit it, in the face of a pureblood, his action was indeed unforgiveable. Right now, even without an evidence to back up Celeste Autumn's words, the fact that Roy had never seen another pureblood after all these years was enough to prove that the human had indeed hunted them to the point that they had to escape to another realm.
Besides, what would Celeste get from lying? There was simply no point in fabricating the story because by the end of the day, she was the only one left from the Autumn family. Without no one powerful enough for revenge, it was nearly impossible for her to do anything.
Celeste was powerless and alone while she had to live on Earth. So, it would be stupid of her to do anything reckless. Because there was no point, therefore the percentage of Celeste telling the truth was higher.