Takemichi having to witness his friends getting killed over and over again every time he returns to the present is horrifying. One must have wondered how this would have affected Takemichi's mental health...
Despite looking like a nerd or a weakling, HE EMBODIES THE MALICE IN EVERY GANG MEMBER, while in comparison, the majority of gang member are more-or-less morally ambiguous fighters who constantly fight over territory.
His obsession with Hina can be said as well. Ever since he met Hina when he was much younger, her kindness ended up attracting the wrong sources, especially Kisaki, who would build his criminal empire over time JUST to impress her. When she fails to return his attention to him, he becomes so enraged that he would kill her in almost any alternate timeline!
Kisaki even pulled a Nightmare Face on Takemichi in the alternate timeline that happened following the Bloody Halloween incident between Toman and Valhalla! After having drugged Chifuyu and Takemichi, he went as far as to mock the both of them before attempting to kill Takemichi!
Taiju Shiba's appearance can be said this as well. Despite the fact that he and Hakkai are capable of pulling a Nightmare Face when provoked enough, the difference between the two is that Hakkai is a nice person who not only befriended Takemichi out of genuine respect for him, but at the same time respects his sister, whereas Taiju is an abusive brother who constantly takes his anger out on his siblings whenever they do something as trivial as forgetting to close the toilet seat! One must wonder how his abuse would have affected his younger siblings...
The fate of Yasuhiro Mutou (aka Mucho) after the Kantou Incident/Tenjiku Saga: While he and Sanzu were talking, Sanzu offered him a ride and took him to the Yokohama 7th pier, where Toman's fight against Tenjiku took place. After a short conversation, Sanzu wields his katana and cuts Mucho's chest in two, with a lot of blood coming out and Mutou's eyes filling with blood from the pain. The page ends with a close-up of Mucho dying, with tears coming out of his disturbingly well-drawn eyes, as he stares into the void and reflects on regretting having betrayed Toman to join Tenjiku.
Sanzu Haruchiyo himself is a classic and well-made definition of this: a drug-addicted, spiteful, vengeful and sadistic psychopath who has no qualms about inflicting the worst kind of torture, beatings and punishments on his enemies, all this combined with his compulsive obsession with Manjirou, his nightmarish faces, the way he usually fights/defeats his enemies make him one of the most brutal and frightening villains in the series. So much so that Manjirou had to ask Yasuhiro to take care of him because of his questionable nature.
And as an addition to his manipulative and psychotic personality, he is also one of the strongest delinquents in the book: in earlier arcs, he, managed to defeat an entire delinquent gang after learning that they had offended Manjirou, was able to almost without any effort or damage, knock out Senju, was able to take down Taiju (who a few chapters ago, had destroyed the entire elite Kantou Manji effortlessly) with just one blow from his katana (all this after almost being crippled after being hit by a motorcycle, being beaten up by Kakuchou and being aremessed from a moving train).
The chapters 253 & 254 delight us with a "beautiful" alternative ending of what would happen if Kakuchou and Takemichi couldn't stop Sanzu from connecting the Freight Yard trains in time.
