WARNING: INTENSE SPOILERS FOR THE DANGANRONPA SERIES.
Chiaki was Hajime's closest friend in Hope's Peak Academy. One of the reasons he participated in the Izuru Kamukura Project was to be worthy of her.
The two first met at a water fountain near the school, when Chiaki bumped into Hajime while playing a game. Since then, the two continued to meet with each other and play games after school at the same place. It is implied that the two of them had romantic feelings for each other, as they often blush when coming in contact with each other.
Chiaki once comforted Hajime after he felt upset about not having a talent. She told him that there's more to life than talent and that making memories with people is more important. Her words comforted him, though his obsession with having a talent didn't lessen.
Before the Izuru Kamukura project, Hajime finds Chiaki by the water fountain, and she invites him to play a game with her. Hajime refuses and says he has to be somewhere. Before leaving, Hajime tells Chiaki that she's more than just an Ultimate Gamer, and tells her to make many memories at Hope's Peak. While walking away from Chiaki, Hajime thinks to himself how one of the reasons he decided to participate in the project was because of his desire to show Chiaki he has a talent. He thought of her while he was being transformed into Izuru Kamukura, and decides to become a version of himself that he can be proud of around Chiaki.
Upon his transformation, Hajime forgot all his memories with Chiaki. As a result, Izuru could not recognize Chiaki, although he confronted her after her execution. After Chiaki dies, Izuru is seen shedding tears despite not knowing who she was, suggesting that a part of Hajime remains in Izuru.
After awakening from the Neo World Program, Hajime, when trying to persuade Ryota Mitarai that hope and despair are both essential, he remembers Chiaki, showing great sadness over her death. While recalling memories of her, Hajime claims that he would never forget her, nor would he want to, and forever thought of her as precious to him.
In the bonus mode of Danganronpa V3, Hajime and Chiaki quickly become friends and often meet up to play games together. She runs late to one such meeting and apologizes, explaining that she had to stay behind as some of her classmates got too rowdy and wrecked it. He says it's no problem, leading to them reminiscing about how well they both get along with their friends. At another point, they're playing Gala Omega when Monokuma pops up to 'supervio clean up her classroomse' them, claiming it to be unwholesome for a boy and a girl to be alone together. While Chiaki doesn't pay him much mind, Hajime expresses annoyance at the bear. In their third year, she extends him an invitation to her class's Christmas party, and he accepts. There, they talk about their plans after graduation, and Chiaki says she hopes they can all keep meeting up like this in the future. Hajime asks her if she's free New Year's Day, and if so whether she'd like to come to his birthday party. She eagerly agrees, and offers to extend the invitation to the rest of her class. He questions whether they'd really want to come, but she says they all like him, so she's sure they will. Chiaki blushes as she says she's looking forward to spending time with him, and he agrees, though he adds it's a bit embarrassing to be making a big deal out of his birthday like this.