Big-Lipped Alligator Moment: Arguably, Tail grabbing and kissing the rat in the fifth volume. This could just be him trying to get over his phobia of rodents, though...
Fridge Brilliance: Abee reacts rather violently to Tick Tock, most especially after she says she cuts people's arms off for not answering her questions or for lying to her. Thus far he had not been this ruthless with anyone else, showing mercy even to Number Three, and Mara calls him out on his willingness to get rid of Tick Tock. More than likely, however, he's remembering his own torture at the hands of Gayle's men, who brutalized him for not being able to answer their questions.
Jerkass Woobie: Number Fifty. His reason for being so outrageous and hateful is that he's never been treated with any amount respect in his life, even being denied the privilege of having a true name. On top of that, the one person who he fears above all else turns out to have been looking for a reason to kill him for years. And just when he thinks he's going to be alright and able to live out his life peacefully, Number Three, his aforementioned worst fear, pops up and brutally murders him.
Tear Jerker: Indigo and Selluriah's deaths, and Mara's reaction to it.