Noah S (previously Admin Noah S) was dubbed "The Mysterious One" because he hasn't been sighted (by other agents) for a long time. He has never been a very public Admin, unlike the Nutty One himself, Gavin B. He worked on the polls on the 39 Clues website. Noah is famous for stealing Admin Gavin B's dentures because one of his desk legs was "far too short for his liking". He was usually the one in charge of book visitations.
Sadly, he no longer is affiliated with Scholastic in any way via. his Twitter account when he messaged ReadingLavender9. Through this he can now no longer be called an Admin (Noah himself stated that he 'USED to be Admin Noah'), he is still considered one among the agents of the MB.