In Chapter 4, Hajime arrives in time to prevent Sato from killing Natsumi, but it doesn't go easily, with Sato breaking at least two of his ribs. Then when she tries to swing the bat at Natsumi's head, he throws his left arm up and blocks and deflects the blow, getting his arm broken in the process. Keep in mind that Hajime hardly knew Natsumi, having only met her a few days prior and considered her an annoyance at best and certainly didn't consider her a friend.
Part 2:
Hajime vs. Hideki. A random highschooler against a trained Yakuza warrior. It ends with Hajime beating Hideki to a bloody pulp and only not killing the bastard because he redirected his knife at the last moment.
Crossing over with Nightmare Fuel at points, in Chapter 12, Hajime escapes his bonds with help from Takuo after they were captured by Korean gangsters on a mission to recover money that went bad. When their original target, Funaki, enters the room they're held in, Hajime puts a scalpel to his throat and tells him that he's going to help them escape. Funaki scoffs at that, pointing out that the Kuzuryu will kill him anyway and then brags about his crimes. After this, he tells Hajime that he doesn't have the balls to kill him before being Killed Mid-Sentence as Hajime slices his throat. Hajime then vomits in a corner over having killed someone. However, he still has 2 other Korean mafia thugs to deal with upstairs. He grabs the only weapon that isn't a small one, a common hammer. Surprising one thug, he cracks his skull with it, incapacitating him. The other gives Hajime a good fight, disarming Hajime at one point and strangling him. Hajime then sees the hammer within reach and clocks him on the jaw with it and gets up. Hajime then clocks him on the knee and then the forehead before brutally beating him to death in an adrenaline-fueled rage. Unsurprisingly, Hajime ends up in a Heroic BSoD after this.
Chapter 13 has 3 guys harassing Natsumi as Hajime is using the bathroom. When Hajime shows back up, they leave, until Natsumi insults them, calling them "thirsty gutter toads". They come back and threaten them and Hajime pushes Natsumi back, inadvertently tripping her. He then proceeds to show off the results of his training, by nearly effortlessly beating up all 3, while they are only able to land one glancing punch to Hajime. When he asks Natsumi after the fight if she's alright, the assailants realize who she is and immediately start groveling and begging for forgiveness. Hajime then tells them they have 10 seconds to leave and they immediately sprint off. Natsumi, meanwhile, was staring in awe at Hajime. Their relationship upgrades shortly afterwards.
In Chapter 15, Juzo Sakakura confronts Hajime and says the headmaster wants to see him and that he'll use force if necessary. Hajime then snarks about the school's already poor reputation and it won't get any better if they start roughing up students. Juzo grabs Hajime's shirt and threatens him, while he remains utterly un-intimidated. Juzo breaks off first when Chiaki tells them not to fight. This is very cathartic when you remember the verbal and physical beatdown that he gave Hajime in the OTL.
Part 3:
In Chapter 8, Hajime is forced to fight his mentor Takuo, who intends to use the killing game to seize power over the Kuzuryu and Minamoto clans. After an intense battle, Hajime manages to best Takuo, ending in an offscreen Off with His Head! moment.
