Awesome Music: Kodoku no Hikairi the ending song of the anime, which translates to "Light Of Loneliness" which fits the feeling of Yako's purpose in life being filled by Neuro very well, as well as describing Sasazuki, HAL and Sai's loneliness
Dirty, the opening of the anime, captures the action and insanity of the series.
Cult Classic: A moderately popular in Japan, the only things it released state side is a Viz license on the anime and not a dubbed version. That being said, it does have an interesting concept on the Monster of the Week storyline being necessary and the depths that Card-Carrying Villain-y can sink to.
HAL and especially Sicks, a rare example that counts as a Complete Monster as well
Mind Game Ship: As pointlessly cruel as Neuro is to Yako, it almost comes across as affectionate as the series goes on.
Judging by the demonic date sim Neuro once played, pointless cruelty is his way of showing affection. And of course, Yako appears in Neuro's wet dream right next to the ultimate puzzle once.
Moe: Miyasako Mutsuki, a little girl who wants Yako to help her grandfather.
Moral Event Horizon: Sicks was never a nice person but what REALLY seals the deal is how he murdersSasazuka by having Sai briefly inflict Shape Shifter Guilt Trip by shifting into the forms of his family before finally shooting him himself...
It's then revealed that Setsuna's degenerating brain was caused by one of Sicks' drugs, thus is responsible for Harukawa's Freudian Excuse
Hell, he may very well have set a record in how fast he crosses it. Namely, slitting the throats of a nursery full of newborns with a razor blade hours after he was born.
DR crossed it when he willingly causes a flash flood, killing off a large percentage of the population of the city where Yako lives. Even Neuro with his Thou Shall Not Kill policy, thought that he should die for doing that.