Facing Mephisto's so-called 'invitation', Roy's first reaction was vigilance.
Although Mephisto was not someone like the King of Lies Belial or Kil'jaeden the Deceiver, it would be a huge mistake if he really thought that the King of Hatred would not lie. It was hard to say if one in ten sentences an old 'demon' like Mephisto said was true.
Therefore, Roy did not intend to answer his question at all. "I have my own plans on which world I want to go to, so you don't have to worry about this!"
His words were not polite at all, but Mephisto was not annoyed at all. He continued to smile amiably and said, "Young man… as a new demon king, it's not too much for me to call you young, right? After all, I only have the qualifications to boast about being a little bit older."