Chiriko Tsurumi, known by her nickname Tsuruko, is a core member of the childhood friend group called the Super Peace Busters. As a teenager, she is tall for her age with a slim build, standing at 170 centimeters. She has straight, mid-length dark blue hair, usually worn down or occasionally in a ponytail, and green eyes framed by red, rimless glasses that she did not wear as a child. Her typical outfit consists of a white collared shirt, a red tie, and a dark blue V-neck tank top as part of her elite high school uniform.

Tsuruko is defined by her serious, logical, and analytical personality, often serving as the quiet observer of her friend group. She has a calm and reserved demeanor and can be highly critical of others, particularly scolding her friend Naruko for her tendency to follow the crowd without forming her own opinions. Beneath her stoic and sharp-tongued exterior, she hides deep insecurities and unspoken emotional pain, feeling that she is a terrible person who only causes problems for others. Academically, she is exceptionally diligent, attending the same elite school as her friend Atsumu and ranking fourth in her class, though this requires immense effort on her part as she struggles to keep up with his natural intellectual gifts.

Her primary motivation is tied to her unrequited love for Atsumu Matsuyuki, also known as Yukiatsu. Tsuruko has harbored feelings for him since childhood but is convinced that he will never love her back because she cannot compete with the memory of their deceased friend, Meiko Menma Honma. Her actions throughout the story are largely driven by this love and a desire to support him, even as she resents his inability to let go of the past. She plays the role of his voice of reason and guardian, understanding his deep guilt over Menma's death and correctly suspecting that his habit of cross-dressing as Menma is an unhealthy coping mechanism. In a poignant symbol of her hidden affection, she keeps a hairpin that Atsumu threw away after being rejected by Menma as a child, and she secretly wears it when she is alone. She also has a habit of sketching while idle, creating drawings that include Atsumu wearing Menma's sundress, which expresses her complex feelings about his obsession.

Within the fractured Super Peace Busters, Tsuruko's key relationships are central to her role. Alongside Atsumu, she is one of only two members who remain close friends as the years pass, and they are together so often that their classmates mistake them for a couple. Her relationship with Naruko Anaru Anjou is initially tense and critical, as she scolds Anaru for being a follower and feels a rivalry with her, believing Anaru could become a replacement for Menma in Atsumu's heart while she cannot. Despite her outward indifference and doubt regarding the ghost of Menma's return—questioning the spirit's true intentions and whether she has truly forgiven everyone—Tsuruko is deeply saddened by her friend's death and ultimately loves her. She joins the group's effort to help Menma achieve nirvana, driven by her feelings for Atsumu and her belief that he is irreplaceable to her.

Throughout the series, Tsuruko undergoes a notable if subtle development. She is forced to confront her repressed emotions and selfish reasons for helping Menma, and in the end, she chooses to act purely out of love for her deceased friend. A significant act of development is her decision to cut her long hair, symbolically letting go of her childhood attachments and accepting the present. In the series finale, she and Atsumu begin to open up to each other, and he gives her a hairpin, signifying a potential step forward in their deeply complicated relationship. Beyond her sharp intellect and observational skills, her notable ability lies in her emotional resilience and her function as the group's anchor to reality, always providing a logical perspective even when it is harsh or painful to hear.