Sumire is an aspiring novelist and K’s love interest, though she does not return the desire. She is in love with Miu, who cannot feel desire for her. In this aspect of the narrative, K and Sumire act as foils, experiencing the same emotions but expressing them in completely different ways. Like K, Sumire is introverted, focusing entirely on her writing and future career as a novelist. With most of her energy going toward this dream, she fails to develop other aspects of her life: “Instead of things I’m good at, it might be faster to list the things I can’t do. I can’t cook or clean the house. My room’s a mess, and I’m always losing things […] I’m bashful for no reason, and I have hardly any friends to speak of” (23). Sumire focuses so much on her own personal goals that she neglects other aspects of her life. She fails to cultivate a social life outside of her friendship with K and barely takes care of herself or her space. This demonstrates how powerful passion is in her life, and how it is restricted to one goal at a time. At the beginning of Sputnik Sweetheart, this passion is for writing, though it quickly changes toward a passion for Miu.

Whereas before Miu, Sumire’s passion was for writing, the introduction of Miu into her life disrupts it by shifting her focus. Unlike K, who is afraid to disrupt his relationship with Sumire, she dives into her love for Miu, accepting a job with her, learning a language, and traveling around the world. With her energy now wrapped up with Miu, Sumire laments her time as a writer: “What really upsets me is I don’t have confidence anymore in the act of writing itself. I read the stuff I wrote not long ago, and it’s boring […] I feel awful, realizing all the time and energy I wasted” (49). Miu’s presence alters the trajectory of Sumire’s life, and her single-minded focus forces her to abandon writing, shifting her perspective on it to one of wasted time, her confidence in her own skill evaporating. This sudden change reflects Sumire’s greater lack of confidence in what she wants to do with her life, a lack that was formerly expressed through her inability to finish a story. 

Like K, Sumire proves to be a dynamic character—when she briefly returns from her disappearance, she comes as a changed friend to K. Though their relationship is platonic, the two friends depend on each other, finding value in the other’s support and presence. Though Sumire does not feel desire for K, her extended absence shows her how important he is to her life: “When I couldn’t see you anymore […] It was as clear as if the planets all of a sudden lined up in a row for me. I really need you. You’re a part of me; I’m a part of you” (209). When Sumire calls K at the end of the novel, seemingly returned from her disappearance, she admits that she needs K. She feels as though the two are bonded and are essential for each other’s happiness. This shift demonstrates a development in Sumire’s character, as she feels confident and at peace with what she wants for the first time, able to focus on multiple passions that now include her friendship with K.

Miu is Sumire’s love interest and acts as a guide to both Sumire and K. She is older than both of the primary characters, and just as Sumire cannot return K’s desire for her, Miu is also unable to reciprocate Sumire’s desire. Whereas both Sumire and K undergo changes throughout Sputnik Sweetheart as dynamic characters, Miu stays static. This difference is rooted in the characters’ different interactions with unreality. Both K and Sumire experience the line between reality and unreality blur during the events of the novel. Miu, however, experiences this earlier in her life, changing who she was previously to the person Sumire and K know: “The person here now isn’t the real me. Fourteen years ago I became half the person I used to be. I wish I could have met you when I was whole—that would have been wonderful” (47). When Miu saw her double having sex in her own apartment from a Ferris wheel, she lost a part of herself and the color from her hair. Miu believes that half of her now lives in another world, having taken her ability to love and desire with her. Her experience and acceptance of it foreshadow how both K and Sumire will have to accept and incorporate reality’s ambiguity into their lives.

Miu is a foil to Sumire in many ways. Where Sumire is lost and disorganized, Miu is purposeful and motivated. She knows who she is and how to carry herself, acting confidently and even spurring a similar change in Sumire. As Sumire grows closer to Miu, she describes Miu to K as though she is anything but an ordinary person: “She’s an absolute master shopper […] Like taking a bite of the tastiest part of a dish. Very smart and charming. When I watched her select some expensive silk stockings and underwear I found it hard to breathe” (72-73). Using a simile to describe her shopping as though savoring the best part of a meal, Sumire shapes Miu’s identity as a woman who is an expert in the finer aspects of life and confident that she knows what she wants. Sumire feels drawn to Miu, and many of the reasons she cites for loving her are the exact opposites of how Sumire herself lives. She knows nothing of fashion or cuisine, staying inside to write, whereas Miu ventures around the world, chasing the world’s finest offerings and experiences.

When K joins Miu on the Greek island, he finds a document written by Sumire that details Miu’s past. He reads the account of how Miu believes she lost half of herself to another world, some imitation of their own: “I was still on this side, here. But another me, maybe half of me, had gone over to the other side. Taking with it my black hair, my sexual desire, my periods, my ovulation, perhaps even the will to live” (157). Miu feels changed after this incident, and though she cannot pinpoint the differences precisely, her hair is the physical legacy of this change. She sees herself as incomplete, with important aspects of her appearance and personality now gone, but ironically, this experience seemingly inspires Sumire’s search for this other Miu, who may be able to reciprocate Sumire’s love and desire.

Carrot is K’s girlfriend’s son and a minor character who appears in the final two chapters of Sputnik Sweetheart. Like K, he is introverted and struggles to forge relationships with his peers. When he steals from a store, K sees how disconnected the boy seems from the world around him: “Carrot remained sunk in silence. His expression and the look in his eyes were unchanged from when we were in the security office. He looked completely blank, like he was going to be that way for a while” (192). In the security office, Carrot is silent and distant, and even afterward, when K speaks with him alone, his appearance does not change. In Carrot’s eyes, K sees a familiar disconnection that mirrors his own, allowing him to see how his own feelings manifest outwardly.

Despite his small role in the novel, Carrot is a dynamic character. After K’s time with him, Carrot changes. By connecting with Carrot on a personal and emotional level, K believes they both benefited: “I believed that telling him the feelings I held inside was the right thing to do. For him, and for me. Probably, more for my sake. It’s a little strange to say this, but he understood me then and accepted me” (203). As both K and Carrot experience similar feelings in isolation, K sharing his own emotions and experiences provides an opening for Carrot to feel less lonely while also highlighting what K has learned over the course of the novel about connection.