Well it appears I may be right (something I always enjoy) about Mamoru! I suggested he was the next most likely character to become a fiend, postulating that his skittish nature, and fear of being eliminated by the Ethics Committee for being the ‘weakest link’, as it were, could cause him to lash out, using his cantus as a weapon. And whilst that has not yet happened, the Ethics Committee certainly seem to be thinking along the same lines as I! It is not disobedience that makes him a threat, he is actually very compliant, but his intense fear of the world around him.
I wouldn’t say I felt particularly strongly about any of the characters in Shin Sekai Yori, but Mamoru is certainly one of the more agreeable characters- there’s just something so tragic about him! There’s his weak cantus, his one-sided adoration of Maria, which Saki, his friend, has openly mocked during a previous episode, his being excluded from his friends’ discussion (he’s much too “weak” and “fragile”), and now his being targeted by the Ethics Committee, not because of something he’s actively chosen to do, but because of something he can’t do a thing about, his fearful personality. Even his attempt at escape from the village was a little tragic- choosing to rely on a tiny children’s sled to carry him to safety belies his childlike helplessness. And just look at those giant brown eyes scream “Help me, I’m so vulnerable”! They’re large enough to make him seem far younger than he actually is, making you want to protect him (which is an effect that has been scientifically proven, by the way)… Yet, at the same time, you almost want to cut out the middle man, and just put him out of his misery right there and then!! I’m interested in finding out if others find him equally as tragic, or is it just me relating to aspects of life I’ve also experienced, or my maternal instincts deep, deep, DEEP down making me fawn over him and his beautiful baby eyes…?
At any rate, Mamoru lived to star in another episode, which is good and all, but who do we have to thank for that? Who was it that stopped the filth cat from tearing him to shreds? It obviously wasn’t the Ethics Committee, because, from what we’ve been told, they planned to kill him off just then. Besides, it would only be counter-productive to let the boy see the filth cat in its entirety, letting him know that he was being hunted by it, then give him a chance to escape, or tell everyone else what was going on! I can only think of a handful of people who might have been able to send the filth cat away: 1. The priest, Rijin, from episode 4- it was never confirmed whether or not he died, and he was one of the two people I’ve seen manipulate air using cantus (but then, why would he help Mamoru?), 2. Maria- she’s the second character I’ve seen manipulate air using cantus, and of course she’d want to save Mamoru (but why would she hide that she’d saved him?), 3. Shun- providing he is not also dead- he’s certainly powerful enough to create whirlwinds and use one to deal with a filth cat. Unfortunately, none of these theories are particularly strong, so I look forward to the filth cat’s banishment being explained, or not, as the case is likely to be…
Watashi Wa Bucho!! 
I’ve never really liked Mamoru, he was too meek and by-the-book. Perhaps it’s because he reminds me of what I was like as a child, and looking back, I can see how it has affected me negatively.