Hiroko Asahina is a central character whose fate is deeply intertwined with the protagonist, Ayato Kamina. She is introduced as a classmate of Ayato and Mamoru Torigai within the sealed city of Tokyo Jupiter, where time flows differently than in the outside world. On the surface, she appears to be in a relationship with Mamoru, but her genuine, heartfelt affection is for Ayato, a feeling that predates the creation of their isolated world.

As the narrative progresses, Hiroko's personality is defined by a growing sense of confusion and anguish. She begins to experience terrifying discrepancies between her pure, authentic emotions and the artificial memories implanted within her. This internal conflict intensifies as her true nature as a Mulian, a being with blue blood, begins to surface. She is painfully aware that something is wrong with her reality, yet she cannot articulate what it is, leading to a state of profound anxiety and fear. Her deepest desire is to seek confirmation of her own humanity, famously asking Ayato to verify that her blood is still red, even as it turns blue before her eyes, a plea born from desperation and denial.

Driven by her love for Ayato and a desire to escape her confusing reality, she leaves Tokyo Jupiter with him, and the two share a brief, sheltered life together. During this time, Hiroko is haunted by terrifying visions of herself connected to a Dolem, the giant puppet-like beings used by the Mulians. She keeps a diary to express feelings she cannot speak aloud, documenting her love and her terror. Her role in the story culminates in a devastating tragedy: Ayato is forced to battle a Dolem he believes is threatening Hiroko. Unbeknownst to him, the Dolem, named Vibrato, is intrinsically linked to Hiroko, sharing her life force and pain. As Ayato fights to protect her, every wound inflicted on the Dolem causes Hiroko to suffer the same injury.

In a final, poignant act, Hiroko uses her latent Mulian abilities to project text across the city's lights and electrical displays, creating a desperate message of love for Ayato by writing his name on the buildings around him. Her soul's final cry, "Goodbye," appears across the metropolis as the Dolem she is bonded to is destroyed, leading to her death. This event profoundly traumatizes Ayato, who only realizes the terrible truth of what he has done moments too late. While her story is one of heartbreak and manipulation in the primary timeline, she is ultimately brought back to life when the world is re-tuned. Her key relationships are marked by unrequited love for Ayato, a complicated and deceptive dynamic with Mamoru who is tasked with watching her, and her hidden connection to the Mulians. Her primary ability is a unique form of telepathy or expression, allowing her to project her inner thoughts, pain, and love as visible text across her environment, a silent scream that tragically goes unheard until it is too late.