I had a terrible thought.
The Will of Mana (WoM) is aiming towards creating an ultimate Demon Lord and Hero that are strong enough to destroy the entire planet and Ryuka is likely that ultimate Demon Lord, but that means there still needs to be an ultimate Hero.
What if Van is that ultimate Hero and the reason his heart was beating from looking at Ryuka was actually because the WoM was modifying his body to become the ultimate Hero right at that moment.
Van met Ryuka after she had been possessed by the WoM but before her parents could enact their plan to trap the WoM.
They spent quite a bit of time (a few months I think?) travelling together to get Ryuka back to her parents so the WoM would have had time to investigate Van's personality and possibly make the changes required.