In the chapter 192, when the suppressed memory of her mother's death suddenly breaks into Miu's consciousness ( She got killed by her own husband while trying to protect little Miu).
Chapter 264 - The tragic self-sacrificial death of Sho Kano, together with the scene where he makes Kenichi promise him to protect Miu. Although, it's still questionable if he's really dead, since his body was immediately taken away. But considering his mentor's reaction and not finding him for quite some time....
"Kenichi... you can have the manju... in the fridge." ;_; To elaborate on this, Apachai just self-revived from a FATAL attack and this was considered his last words. Best Big Damn Heroes ever Akisame.
Natsu Tanimoto's whole freaking backstory is just a soul-crushing Trauma Conga Line. Especially the dead sister part...
Kenichi actually losing to Shinnosuke Tuji because he was careless (at this point Kenichi is strong enough to curb stomp him if he took him seriously). His defeated look and the look of disappointment on Sakaki's face really made this troper feel down, not to mention Kenichi losing some of the confidence in himself.
Chapter 465 OmakeTanimoto's face when he heard of Sho's death.
Rimi/Atalante seems to die from using the same technique that cripped Ryuuto. Thankfully, Ogata saved her life at the last moment, but it was tragic to watch her push herself beyond her limits just to impress Ryuuto, especially since Ryuuto was screaming at her to not use that technique.
In chapter 519 Chikage sobs uncontrollably when the conflict between what she was trained to be, a soulless killer, and what she truly is, a little girl, finally becomes too much to bear.
Tanaka's past is rather depressing, being driven by the desire to avenge his murdered master and watching what at first glance appeared to be a perfectly adjusted member of society show his inner darkness. Then it is revealed that Ogata not only killed Tanaka's master and burned the dojo to the ground, he also murdered his wife AND HIS UNBORN CHILD.
Tanaka ultimately dies at Ogata's hands despite all of his efforts. The last bit of advice he gives to Kenichi?
Don't be like me.
This is rendered further into tearjerker territory via "Funny Aneurysm" Moment. A Running Gag of Tanaka's was interrupting the action to speak to his family. Cue this fight when we learn all those times he was calling an automated Test Call phone number.
Combined with a little Fridge Horror, if Miu was killed by Ogata after the fight with Rimi, Kenichi may have ended up like Tanaka.