Parnassus Girl asked:
Kumari de Silva
This answer contains spoilers… (view spoiler)[Both the Other and his teacher (Laurence Arne-Sayles) believed there were deep secrets to the universe and arcane knowledge hidden in the house. Both of them were also exploitive evil people. They each knew the danger of staying in the alternate universe for more than an hour, as they had witnessed people loosing their memories and also appearing to lose their minds. Being in the house made most people despair, desperate, and amnesiac. Piranesi seems to be an exceptional person as he lasted so many years and remained connected by love to animals and things, he also was a journal keeper and very astute. Despite forgetting who he was, he was still really good at reasoning whenever he read his old journals and that saved him. Ketterley and his teacher really missed the mark - - you don't learn more by going deeper and deeper into the House, apparently you learn more by being a compassionate person and that is why Piranesi isn't eager to return to his "dead" name even at the end (hide spoiler)]
This answer contains spoilers… (view spoiler)[I believe that Piranesi was used by The Other to learn more about The House to understand its layout/how that world worked. The Other didn't want to do it himself because he knew that he would lose his memory/go mad much like Piranesi did. That's why The Other and Laurence Arne-Sayles were only ever there for an hour at most because they knew that would happen if they stayed there any longer. I think The Other became annoyed with Piranesi because he was tired of dealing with Piranesi's behavior/lack of understanding due to his memory loss - and he was just a terrible person ;) (hide spoiler)]
Laura After MRS/Piranesi lost his memory and was no longer a threat, he became pretty useful due to his knowledge of the tides and other aspects of the house.