These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Black Cat
Angst? What Angst?: Train in the later half of the manga. He gets shot by a bullet that is known to transform people into monsters, yet just shrugs it off (despite everyone else including the Big Bad panicking). When he turns into a kid, his first thoughts are that it'll make meals and bus tickets cheaper. When asked if he's concerned that Creed is trying to bring about the End of the World as We Know It, he answers no, and that he's mainly concerned with what's for lunch.
Compressed Adaptation: The anime, which removes whole arcs, completely rearranges the whole start of the series, and cuts a lot of character development for everyone but Train and Eve, or distilled in a very awkward way. Which is strange considering that the anime is also an example of Adaptation Expansion.
Complete Monster: The Doctor is more twisted than Creed and doesn't have the excuse of being clearly insane. Pick one of his experiments at random, just one of them, and you'll likely agree that it's enough to warrant the most cruel and unusual death available. He even tells Eve at one point while trying to vivisect her that empathy is a truly pointless emotion.
Tendencies? The man has exactly zero qualms about killing you if you "stand between me and Train." Given his obvious propensity for seeing things obsessively... does anyone here have a mathematical formula for how long it would take for Creed to end up killing the entire world's population out of jealousy?
His lack of eyebrows cannot be excused.
Some Apostles naturally get this treatment. Shiki's nearly morally bankrupt, but s/he's beautiful. Charden has done terrible things on a vengeful crusade, but he's such a gentleman. Kyoko's an obsessive, psychotic pyromaniac, but she's so funny and adorable. Echidna's a conniving bitch, but she's sexy. Durham's a Trigger Happy psycho, but he's so cool. Leon's a budding psychopath himself, but he's such a cute little kid. And the Doctor...wait, even fans know there are no excuses for that asshole.
Also Shaolee, who spends his time deceiving people, is actually one of the cruelest Numbers and is quite creepy when you think about it, but looks just so pretty.
Evil Is Sexy: Echidna, although she oddly never makes use of it. Kyoko too to a lesser extent. As for male villains, Creed, Shiki, Charden, and even the Doctor are very attractive.
All of the Yaoi Fangirls want to see Creed and Train together. (Well except for the small amount that go for Sven/Train.)
While Train/Kyoko has it's own following, Charden/Kyoko seems to be even more vocal.
Fashion-Victim Villain: Nice jumpsuit, Creed. Did it come with the open chest, or did you cut it out yourself?
Foe Yay / Ho Yay - The romantic / sexual tension between Creed and Train. Though it's rather one-sided, as Train clearly has zero intention of returning Creed's affections. In fact, he doesn't even seem be aware of them, which isvery impressive.
It Was His Sled: If you start the series through the anime, what happens to Saya should be obvious due to the huge amount of foreshadowing in the opening and ending.
Creed murders Saya simply for the humanizing influence she has on Train. And after suffering a nasty injury that might have made people sympathize with him, he then tries to turn Sven into a mindless monster.
When facing down Sven, Torneo is all ready to use a nanotech drug on Eve to force her to become a mindless killing machine. Train shoots it out of his hand to stop him.
All of the Doctor's experiments put him far over the line.
Unfortunate Implications: The only obviously gay or bisexual character in this series is The Big Bad, something that can be easily taken the wrong way. That said, there are a few upsides to Creed' portrayal: he nicely subverts the "gay men are wimps" stereotype despite being extremelycampy, is not a pedophile (he and Train are actually around the same age, Creed actually being younger than Train by a few months), and in his own warped way, is very loyal to the person he's interested in. He's depraved in the sense of "gay and evil", not "evil because he's gay."
As an aside, it's also worth noting that his love for Train is actually one of his only redeeming qualities. The only time he ever cries is actually over Train, and the only possibility of a Heel-Face Turn for him was because of his admiration and weighing of Train's words.
In the anime he managed to make a successful Heel-Face Turn… and ended up in a relationship with Echidna. Though since it was after he pulled his If I Can't Have You move on Train he could have just realized that Train will never love him and was able to finally see that Echidna had feelings for him, so you could say he just moved on. That, or they wanted to Pair the Spares.