Aya Asia is a member of the New Bloodline.Unlike most human villains, her Evil Intent within her Id didn't overwhelm her sense of self. Not only that, but keeping herself in the Darkness actually gives her special powers; like the New Bloodline. Her sonic based songs may be an aspect of Air; like how the others have shown powers of Fire, Water, and Earth.
- Her ideal of Loneliness conflicts with the New Bloodline's ideals; she likes resonating with the lonely, but has no desire to kill them off (unless she likes them too much).
Sicks was lying when he told X that he's a cloneWe know that Sicks gets his lulz from telling people whatever will cause the most psychological pain. The most painful thing to tell X after killing his most important person is that he never had an identity. They did experiment on him, though.
X/Sai's powers didn't come from his mother. They came from his father.A blend of human genetics with just a touch of demonic from some distant ancestor. His instability comes from being born within the Mayan Hell Seal; which prevented a crucial psychological link between mind and body.
Yako has the same physiology as X/Sai.Her tremendous appetite is actually her metabolism's way of maintaining her mind and body. In the manga, it's pointed out that X/Sai has the same bizarre eating patterns that she does. It's her mental stability that prevents shapeshifting and amnesia from occurring. X even comments on how he didn't have to reprogram her much when he kidnaps her in the manga; he just altered her metabolism.
- Yako has dodged attacks from the New Bloodline as well as grabbed another person out of the way of a New Bloodline assassin.
- More evidence: Neuro took Yako to a gym and scared her into putting her all into physical exercise. The results were superhuman.
Yako is starting to feed on Mysteries in a manner like Neuro's.At the end of the anime, we see Neuro watching as Yako "absorbed" some of X/Sai's aura, turning it white. Her method, however, purifies.
Neuro is a much younger Tzeentch from Warhammer 40,000After all, Neuro is vaguely avian and definitely alien in his true form, derives sustenance from negative emotions and thrives on the complex situations resulting from them, undeniably one of the most powerful things in Hell/The Warp, when on Earth is determined to mask his presence from watching eyes that might be interested in wiping him out (the Emperor), and of course is the biggest Troll ever...
Sicks was responsible for Setsuna's disease in the HAL arcNear the end of the flashback section, we see one of the doctors describe her disease as being a "mass of evil intent". Also, Sicks' group has been known to experiment on humans. And besides, it would serve to tie together all the arcs.
- Spoilers: Confirmed. Sicks used Setsuna as a test subject while looking for a way to counteract X's constant memory loss, experimenting on her to strengthen her brain and leading to her eventual death.
The New Bloodline are not a special offshoot of humanity as claimed by Sicks.Shortly after Vijaya's death, Godai makes a comment suggesting that he doesn't believe that a special "evil" breed of human could exist, before passing it off as just him being stupid. However, there's no evidence that they really are specially evolved, as all their abilities are purely knowledge or biotechnology-based.
- The knowledge seemed to be a form of Genetic Memory or Lamarck Was Right more than actualy evolution. The latter regarding Sicks himself (the two most evil breed, the offspring is even more evil, does more evil, etc.) and the former with everybody else. Especially given the explanations for X's powers.
Had the series continued, Sicks would be brought back by the powers of scienceSicks was shredded to bits during his final battle with Neuro in the middle of the ocean, its likely some of his followers will revive him in the same way they created Sai.
MTNN takes place in the same 'verse as My Neighbor Seki (Tonari no Seki-kun)This is a very weird pet theory of mine based solely on the fact I think Yokoi looks vaguely similar to Yako. I personally think two things with this impossible cross-over are most likely. 1. IF Yokoi is indeed related to Yako, then they are cousins (probably through Haruka/Yako's mother) and the events of MTNN happened after the events of Seki-kun. I think Yako and Yokoi are the same age and that what's happening to Yokoi is a classroom next to the regular lessons of Yako. But I also have another spin on this theory. 2. What's happening to Yokoi is what's happening in a future which has been made more-or-less safer through Yako's negotation. The students of Japan are now living in a more peaceful era where eccentricity is fine. Yokoi is just slow on the uptake.
The world of Assassination Classroom take place in a universe parallel to Majin Tantei Nougami Neuro.Since Neuro confirmed the existence of parallel universes in the penultimate chapter, Assassination Classroom's setting might as well be one of them — after all, both series are written by the same author. A couple of characters from AC even share similar traits with Neuro's protagonists.
- The prime ministers in both series have the same name and appearance. They're basically the same person.
- Like Neuro, the Chairman is a clever, sadistic, Manipulative Bastard who seems fairly pleasant on the surface.
- AC has two characters that closely resemble Yako. Nagisa, the protagonist and narrator of the story, has the same height, observational skills, and amicable personality as Yako. Fuwa, one of Nagisa's classmates, shares Yako's voice actress and love for mystery solving.
- Terasaka, like Godai, is brash and aggressive but ultimately very reliable when the situation calls for it. Both characters serve as The Big Guy for their respective series.
Neuro is the Emperor of Hell.Although obviously it's not so much of an actual ruling position as it is a title given to the strongest demon, which also comes with the ability to summon the 7 Weapons of the Demon Emperor. When using the Evil Metal against Sicks, he states that not even the Emperor could survive being cut by it, so he most likely used his cunning to finish the former one off with their own weapon. His early claim to Yako that other demons 'humbly bow their heads' when they see him wasn't totally bullshit.