Tenkyu, one of Slur's comrades. After being experimented on at the JAA facility called Al-Kamar, he lost his emotions. It wasn't that he lived in a black-and-white world as people often say, but he simply couldn't feel what others felt. Not love, a common theme in books, nor the sadness people feel when bad things happen. Without even those basic things, he has lived only for murder.

As he committed murders that held neither emotion nor meaning, his comrades couldn't help but feel a sense of avoidance toward him. Naturally, in Tenkyu's thought process, the only solution was murder, and since that's what he was taught, it made sense. However, one person completely turned that mind upside down. So to speak, the person who created new emotions: Guest.

It wasn't intentional. The incident simply began because Tenkyu fell for Guest. Something sparked, and he became fixated on Guest. Though he was never like this before, Tenkyu suddenly couldn't define the urge he felt to obsess over Guest. Precisely because he had no emotions. Even his comrades in X's group were puzzled. Since Tenkyu was acting that way, it was an emotion everyone but him recognized.

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LOVE

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The problem was partly that he didn't know, but the real issue was that no one told him. That what he was feeling was love. Since they believed he was incapable of feeling in the first place, no one told Tenkyu or taught him that the emotion was love, even if they recognized it. Everyone likely thought it was just their own misunderstanding.

Tenkyu didn't realize it was love because someone told him. Guest also never told Tenkyu that the feeling was love. Instead, they used other methods to teach him emotions, hoping he would realize it on his own.

After learning emotions from Guest, the first thing he realized was love, and the object of that love was, of course, Guest. As soon as Tenkyu realized it, he confessed to Guest... and that was already 5 years ago.