Hajime uses the favor owed to him by the Kuzuryus for saving Natsumi's life to also spare Sato and her family from any repercussions by the Kuzuryu-gumi from her attempted murder of Natsumi. Hajime points out that Natsumi did a lot to instigate it and that Sato now faces criminal charges and is expelled from Hope's Peak and thinks that that is punishment enough.
Hajime comforting Natsumi while he's in the hospital after her actions nearly got her and potentially him killed by Sato, as well. When she says he probably hates her guts now and he doesn't have to talk to her anymore, he says he doesn't hate her and still wants to talk with her and that just because she made some mistakes doesn't mean she's evil or deserved to die.
Then when she's beating herself up over thinking herself not being worthy of being Fuyuhiko's sister, Hajime "cut through Natsumi's words" and told her that she doesn't need to be an Ultimate or someone special, since she is Fuyuhiko's little sister, she just has to be herself.
A couple days later, as he's still in the hospital, Natsumi brings him a bento box for in a full-on tsundere way.
In a weird way, but when Chiaki comes in with a bento box, and Natsumi sees how close Chiaki and Hajime are, she feels awkwardly jealous and sad. Chiaki also subtly shows she's a bit jealous of Natsumi, pointedly ignoring her and when Natsumi interrupts Chiaki and Hajime's conversation, she gave Natsumi a bored look. It's different than how you'd expect Chiaki to act to a girl who had nearly been killed a few days prior, but Natsumi was saved by Hajime, who she has feelings for.
Mahiru thanking Hajime for preventing Sato from killing Natsumi, rightfully pointing out that she'd be killed in return had she succeeded. Then Hajime talks Mahiru into telling Sato the truth about her feelings, that she likes Sato as a friend, even though Sato is romantically interested in Mahiru. He tells her that Sato has been her friend for years and didn't just stay with her because she was attracted to her. This cheers up Mahiru and she wonders how Natsumi got a great guy like Hajime and Hajime basically says "She's Not My Girlfriend". Then she says "Oh? Is that so? Well then, I guess she won’t mind if I do this” before giving him a peck on the cheek.
As they're walking into school after Hajime gets released from the hospital, after thinking she was a pain in the ass before he saved her life he now thinks to himself of Natsumi:
Right now, the only person who kind of mattered to Hajime in this entire building was walking alongside him. That was a strange thought to consider.
In Chapter 7, Fuyuhiko asks Hajime what his intentions towards his sister are. He then comments that Hajime is closer to her than anyone has been and that her poor people skills meant she rarely got friends and those she got rarely stuck around. He then warns Hajime that if he hurts Natsumi, it's not Fuyuhiko that would come after him, it's Peko. Understandably, Hajime doesn't want that to happen.
In Chapter 9, Natsumi gets upset when Hajime says they don't need a talent or to get into the Main Course. After being distant and angry with him for the rest of the day, when they meet after school, it turns out she was angry because “If we’re giving up, There is no reason for either of us to stay here and… there's no reason for us to talk to each other anymore,". Hajime then tells her that it doesn't change a thing and he's not going anywhere. She then hugs him in an adorably tsundere way and blushes like crazy.
In Chapter 10, Natsumi becomes friends with Aiko Umesawa, a minor character from Danganronpa 3 after clearing up that she was trying to turn over a new leaf and stop being a bully.
Also in Chapter 10, Natsumi apologizes to Mahiru for bullying and threatening her. Mahiru then angrily grabs Natsumi and says that she suffered for years because of Natsumi and asks why now she's able to feel bad for it and why she bullied her while crying. Natsumi starts crying and admits that she was jealous that Mahiru had a talent and friends while she had none of that and that her reasons are as shitty as they sound. Mahiru then slaps her and composes herself for a few moments. Then she moves in and hugs Natsumi and tells her that all she had to do was tell her that she was suffering and that Natsumi's bullying was completely wrong. Then she forgives Natsumi.
In Chapter 11, after it's revealed that Hajime is being paid by her father to be her bodyguard, she gets angry that Hajime never mentioned it and thinks that's the only reason he put up with her for the previous months. To show he likes her for more than just being paid, he kisses her for the first time.
Hajime: “Listen to me Natsumi. I don't care about your dad’s money or about anything else. All I want is to be by your side. I just want you. That’s it. Don't ask me why, because I can’t tell you for sure. I don't have any experience in this kind of thing. All I know is that ever since I met you, I’ve been living, for once. For once, I don't want to be anywhere else other than where I am, right here. Right now," he said, taking her hand and placing it on the left side of his chest, where she could feel his throbbing heartbeat.
From the same chapter, Natsumi partially snaps him out of the BSOD just by the thought of being with her when she suggests she come down to see him. Later, they end up having their First Time together.
In Chapter 14, it shows that Natsumi is now friends with Mahiru and the rest of her group of friends, Ibuki, Hiyoko and Mikan. She's also the most comfortable around Mahiru, who is the most normal of the group. She thinks of Ibuki that "there was scarcely a more innocent and kind-hearted girl in the entire school." despite her looking like a "try-hard delinquent", but also thinks she's a "weirdo", due to her Cloud Cuckoolander tendencies. She gets along with Hiyoko as well as you'd expect. We never see how well or badly she gets along with Mikan.
Ibuki calls Hajime and Natsumi "such a cute couple!" and when Natsumi mentions that Hajime's internship with the Kuzuryu-gumi finishes in just a few weeks, Mahiru suggests that they, as well as Fuyuhiko and Peko, throw a welcome back party for him. Sadly, the Student Council Killing Game occurs and that falls through.
Combining with Moment of Awesome, Chapter 15 has poor Chiaki get her game system stolen by 3 delinquents. However, a post-Yakuza training Hajime shows up and, seeing Chiaki crying, asks her what's wrong and she says that they stole her game system. Hajime immediately goes off to confront them, despite her protests. He confronts them and after some initial bluster, they recognize him as someone affiliated with the Kuzuryus. They immediately apologize and when he asks for Chiaki's game system back they offer to protect her. An irritated Hajime just takes it and walks away without saying anything. When she worries that they may have hurt him, he replies:
