The relationship Yamato has with Koya is a passionate and dedicated one that becomes unexpectedly stressful for Yamato in volume 4. Her marking, which she interprets as physical proof of her relationship with Koya, slowly fades without warning. In a blatant reference to the homosexual nature of their relationship, both girls' markings — a 0 with a line through the middle — are on the same place: their breast. She once invites Ritsuka to a coffee shop, and reminisces the first time she and Kouya met. When she saw what Kouya did to her wrists, she got the cutter and slit her wrists too, in which point Kouya apprehends her. She replies that whatever injury Kouya has, it will also appear on her. She holds her wounded hand to her heart, and says "it's beginning to hurt, here." In the coffee shop, Ritsuka accidentally spilled some coffee on her blouse, a result of being surprised when she said she's going to show him her mark (her breast). She goes in the bathroom claiming it is in order to remove the stain since it's going to spread. Inside, she opens her blouse and stares at her mark, which is beginning to fade, meaning it won't be long before she is not going to be Kouya's sacrifice anymore.
During their fight with Loveless and Soubi, her mark has completely disappeared, so she had to endure all of Soubi's attack. Kouya doesn't know so she proceeds fighting. However, when Yamato is brought on her knees by the pain, Kouya begins suspecting, in which Yamato vehemently refuses and says "it doesn't hurt!" Kouya forcibly opens Yamato's blouse, and sees that the zero mark has completely disappeared. She withdraws from the fight, even if it means certain outcomes. They decide to die together and be reborn than to live separately.