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.
Kakihara, a twisted sadomasochist whose boss is murdered by the titular Ichi begins a hunt for the mysterious assassin in his obsession to discover the ultimate pain. Kakihara participates in a number of murders of Yakuza and civilians alike and has no compunction allowing rape or torture to occur under his watch. Cold-Blooded Torture is his specialty, in fact and he tortures one man by hanging him from multiple needles and hurling hot tempura oil over his bleeding back. Obsessed with the point where pain and pleasure meet, Kakihara terrifies all who meet him.
The Creepy Twins and Co-Dragons to Kakihara, Jiro and Saburo, brag they've raped hundreds of women. Driven to outdo one another, they introduce themselves by asking a pair of lovers who the women preferred in bed. When the women give an answer, the offended twin viciously tortures and kills them. The two later abduct a prostitute after brutalizing her and rape her to get her pimp to talk. Despite Kakihara's orders not to kill them until he verifies their information, the two play a game to see who can hurt the two worse solely to prove who's better at causing pain. When one accidentally kills the pimp, the other kills the girl just to prove he can kill someone as well. After Ichi kills Jiro, Saburo is only angry because he wanted to kill his brother himself one day. He then proceeds to kill a man Kakihara tortured earlier and rapes and murders the girl with him, before killing her with a knife through her genitals.
Crosses the Line Twice: For the most part the manga series just stay past the line, dark, gritty and brutal, but then you get to the point where a crying Ichi chases after a nude, bleeding and split-dicked masochistic mafioso and the scene gets described as "romantic", framed in a bubble shot and all at that point it just swings around to twistedly funny.
Designated Hero: Ichi, Jijii, or Kakihara - pick one, then designate them the "hero".
I Am Not Shazam: Kakihara is usually the one shown in advertising of the comic and movie - sometimes they even put a "1" on his face. The titular Ichi, on the other hand, is a sobbing weakling wearing a stuntman suit.