I've been... talking to Professor Valvatorez. He thinks it's Zarc, too, and Serena saw one of his memories. So, it has to be...

[Well, if they already thought Zarc was in Yuya, then it stands to reason that the memory Serena saw certainly came from him. He has the Demon King possessing him, and that's why he can't relax and accept 'maybe nothing will happen'. He was a monster.]