The so-called AB blood type is defined based on the presence or absence of specific antigens (agglutinogens) A and B on red blood cells—a classification system that emerged due to advances in medical science, or more specifically, the needs of blood transfusion.
Based on the distribution of A and B antigens, blood is divided into four types: A, B, AB, and O. Red blood cells with only antigen A are type A; only antigen B are type B; both A and B are AB; and neither A nor B is type O.
Patients who lose a significant amount of blood require transfusions, but if the blood types don't match, the transfused red blood cells can clump together, causing vascular blockages and massive hemolysis—potentially life-threatening.
For this reason, blood typing is essential.
As for why blood type is relevant to determining parentage, that comes down to Mendel's famous laws of inheritance.
Blood type is controlled by three alleles: A, B, and O. A and B are dominant, while O is recessive.
Both of Fujiwara Chika's parents are type O, meaning genetically they are homozygous for the recessive gene and carry no A or B alleles. In that case, none of their offspring could possibly be type A, B, or AB.
And so, Chika's dilemma was—
"Hara Kei, I… I think I might not actually be my parents' child. What should I do?"
Just like that.
It was an unexpectedly heavy topic.
Hara Kei clicked his tongue and glanced at the pocket of Chika's uniform; he had seen her place that silver key inside earlier.
Judging by its size, its fit with the rooftop door lock, and a bit of logic, he was fairly certain—that was the key to the rooftop.
Which raised the question—why was Fujiwara Chika heading to the rooftop?
If this had happened before, Hara Kei would have assumed she was going to water the plants or enjoy the view. But now… he wasn't so sure.
Rationally speaking, Chika wasn't the type to do anything reckless.
But now, she was in that most self-conscious, rebellious phase of adolescence—and facing something like this…
Better to be safe than sorry.
Chika hadn't noticed Hara Kei's suddenly serious expression; she was venting her emotions aloud.
"To be honest… I really don't know what to do anymore." Chika had let go of his jacket hem at some point. She hugged her knees tightly, clutching her skirt with trembling hands as though trying to physically restrain herself.
"I can't tell anyone. Our family… if word got out, it would easily become a scandal, used to attack the Fujiwara family politically."
"I also can't ask my parents." She had managed to steady her hands, but her voice still trembled uncontrollably. Hara Kei could tell immediately that it wasn't from the cold.
"And if I did ask… what would happen? What if they dropped the facade of kindness and told me that everything I believed was a lie? What would I do then? Be shaken? Cut ties? Tear apart our relationship? Lose control?"
"And if I don't ask… what then? If I do nothing, say nothing… is it better to just keep being the 'good daughter' and pretend I know nothing?"
"I've tried living like that for days… it's exhausting." Chika's gaze focused somewhere in front of her, but her eyes were empty, hollow.
"At times, I think—my parents care for me deeply. They've always protected me, fought for me whenever there's trouble. I'm sixteen now. Even legally, I could live on my own. Out of respect for the family, shouldn't I just… not dig any deeper? I've lived like this for over a decade anyway, right?"
"Rationally, I try to convince myself… but unfortunately, humans aren't purely rational beings." Chika let out a small, helpless laugh—Hara Kei had never seen her smile like this before.
"If it were you, Hara Kei, confident and independent… you'd ask outright. Even if it meant standing alone, you'd seek the truth."
"That courage… it's enviable."
"But me? I'm still just a coward."
Chika exhaled deeply. "Saying all this… it feels a little lighter."
A lie. Her slightly trembling body betrayed her words.
"You can't tell your friends, but you're pouring your heart out to me? Not very smart, Fujiwara," Hara Kei said, his tone laced with dry sarcasm. He wanted to gauge whether she was perceptive enough to understand her own feelings; if she were completely oblivious, he might have to intervene physically—he couldn't just watch her head to the rooftop.
"Because I trust you, Hara Kei," Chika replied immediately. "You've never really treated me as a friend, but you're the most trustworthy person I've ever met. I don't think you'd go blabbing this to anyone else."
…Flattery wouldn't work on him now. But at least her reasoning was sound; the situation wasn't beyond saving yet.
"Fujiwara Chika, what do you want to do?" Hara Kei asked in his usual calm, measured tone.
"Eh…? Uh… I'm struggling because I don't know," Chika said, pressing her hand to her chest as if trying to locate her own feelings—but failing.
"Then let me ask it differently," the boy asked slowly, deliberately, word by word, giving her time to think carefully. "What outcome do you want?"
"…Maintain the status quo? …No…" The girl forced a bitter smile. "…I just want to be wrong. I want to truly be my parents' daughter."
"Isn't wanting that… naive? Immature?"
"No, I think that's only natural," Hara Kei shook his head. "And in my view, that desire is neither naive nor immature."
"In fact… I think it's the most likely truth."
"Eh?" Chika's eyes widened in surprise.