Majin Tantei Nougami Neuro/WMG

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 clone
We 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 has recently taken 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.

Sicks was responsible for Setsuna's disease in the HAL arc
Near 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:

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.