Complete Monster: The anime version of Kaneyuki "Urado" Miyama was a sickly man trying to extend his life in the worst way. Changing his name to "Urado" after Vlad the Impaler, he began to abducts countless innocents, cutting their throats to bleed them out and bathe in their blood. After his death, his ghost continues haunting his old home, abducting those who investigate or wander inside to murder them for their blood as well. When the heroes arrive with other paranormal investigators, Urado wastes no time in abducting and murdering several of the investigators before trying to murder the heroine.
Some fans also seem to like Masako and John together; an odd pairing given that the former only has eyes for Naru in canon and the latter is a Catholic Priest.
Rounding out the list is Lin/Madoka, which is very slightly teased in canon when Mai wonders about their relationship to each other at the beginning of the manga's Bloodstained Labyrinth arc.
Informed Wrongness: Mai gives Naru a What the Hell, Hero? in "File 6: Forbidden Pastime" for wanting to turn the curse back on the students, potentially hurting or killing them in the process to save a thoroughly horrible person who doesn't deserve to get off without consequences, but is told to 'trust Naru' and ultimately is treated as needing to apologize for the outburst because he actually used dolls to absorb the curse without hurting anyone. This is despite the fact Naru made no attempt to actually tell her, something that nearly directly results in her death for no real reason and from her perspective, that was exactly what he was trying to do.
Tear Jerker: The episode Silent Christmas. "Kenji made it home for Christmas..."
Also the ending of the Doll House storyline. In a good way. Happy tears.
Gene's death. Hearing it described it is terrible enough, especially since it's revealed in the final volume, after so much build-up to create our attachment to the "kind Naru". Then you remember that Naru, his twin brotherfelt and saw every second of it. But the worst thing is you can finally know for sure that Mai will not have a chance to be with him.
Unintentionally Unsympathetic: "File 6: Forbidden Pastime" arc tries to paint Mai blowing up on Naru for turning the curse back on the students as being out of line and something she needs to apologize for because they were going to use dolls to transfer the curse to and he never intended to hurt anyone. The issue is Naru refused to tell her about that, something that nearly gets her killed, making Naru come off as entirely in the wrong, and he only has a weak explanation for why he didn't. It doesn't help this is all to help a monstrously awful person whose cruelty caused the whole mess and drove a student to suicide and Naru doesn't face any consequences for this.
