In the first castle, there was a gap that seemed above Queen Jane's room and below the person who walked right and left. Check the lower right corner of this area to find out if there was an invisible person here. Talk to him and he'll tell you something before and after you save Princess Sarah.

•  There's a castle with Queen Jane's room on the first floor.
•  Above is a gap in one position where a person walks around (probably a guard or a patrolman).
•  In the lower right corner of the gap is an invisible person.
•  He only appears to be hidden and exposed on closer examination.
•  He has information to share that might be relevant to the story.

 Before saving Princess Sarah.

•  He can offer mysterious clues or advice about a princess' position or how to overcome obstacles.
•  He may warn you of danger or betrayal within the castle.
•  He may be a spy or a prisoner with valuable knowledge.

 After saving Princess Sarah.

•  He was able to express his relief and offer additional help.
•  He might reveal his identity and explain that. Why can't he see?
•  He can offer more missions or knowledge of castles and people here.

 This invisible character has the potential to be a curiously interesting element in the story behind these characters. 

•  Consider why this person can't see it as a magic curse or a talent or a technology device.
•  Explore that person's motives, why they help you, are they loyal to the Queen Princess or have their own agenda?
•  You can add additional tasks to help break the invisibility spell or expose the person's past.