Step 1: Have them listen to the Chronicles version of To Make The End Of Battle:
Step 2: Tell them that the Ys games are as awesome as that song ALL THE TIME.
Step 3: This person will want to play an Ys game.
This is a tried-and-tested method to get someone interested in Ys.
(Note: In case of the .0000000000001% chance that this method will not work, have this person watch you play Ys II and show them how enemies explode into chunks when you defeat them. If neither of these methods work, then the person you are trying to convert to the Ys religion is already beyond salvation.)