Hajime Usami is the main protagonist of Double-J and a first-year student at Nagashima High School. She is the primary character through whom the audience experiences the school's unusual after-school activities. Her introduction to the story begins as a matter of necessity, as her school mandates that all students must participate in a club. She, along with her close friend Sayo Arima, discovers and decides to join the Traditional Art Inheritance Club, also known as the Cultural Activity Preservation Club.

In terms of personality, Hajime is depicted as having a somewhat easygoing and unhurried temperament. She is not an exceptionally loud, aggressive, or standout character, which places her in a narrative role similar to a straight man or an everywoman surrounded by more eccentric personalities. She does not initially possess any particular skill or deep knowledge of traditional Japanese crafts, nor does she initially put forth a great deal of effort to join the club, but she decides to stay and observe anyway. Her motivation for being in the club evolves from a simple requirement to a genuine, if quiet, interest in mastering a classic art form as she spends more time with the group.

Within the story's structure, Hajime serves as the anchor for the viewer. She is featured on the cover of the first volume of the manga, signaling her importance to the narrative. Her role within the club itself develops over time. While she starts as simply a new member, she eventually takes on more responsibility, succeeding the vice president, Aya Chosokabae, after that character graduates. This progression shows her shift from a passive participant to a more integral part of the club's leadership.

Her most significant relationship is with her friend Sayo Arima. The two are often found together, with their contrasting personalities creating a common dynamic; Hajime is frequently associated with the "blue oni" archetype, which is typically more calm and reserved, while Sayo represents the more energetic and impulsive "red oni". She also interacts with the other club members, including the eccentric president Ichiro Toba, who claims descent from a famous artist, and the senpai known as "Toothpick-san" for her dedicated craft. While Hajime is not defined by flashy abilities, her key development is her growing investment in the club. She moves from a state of indifference to finding personal value in the preservation of traditional arts, culminating in her taking on the vice presidency. Her notable ability is not a specific craft but rather her function as the calm, observant core of the group that allows the other members' quirks to shine.