Tenkyu, one of Slur's comrades. After being experimented on at the Al-Kamar facility created by the Order, he lost his emotions. It wasn't like living in a black-and-white world as others often say, but he couldn't feel what others felt. Not love, a frequent theme in books, nor the sadness people feel when bad things happen. Without enjoying even those basic things, he has only known murder.

The more he committed murders that lacked emotion or meaning, the more his comrades tended to avoid him. Naturally, the only solution in Tenkyu's thought process 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 started because Tenkyu fell for Guest. He was struck by a flash of something and became fixated on Guest. Even though he was never like this before, Tenkyu suddenly saw his own desire to obsess over Guest and couldn't define what it was. 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 except 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 you're feeling now is love. Since they believed he couldn't feel in the first place, no one told Tenkyu or taught him that the emotion was love, even if they knew. Everyone probably thought it was just their own misunderstanding.

It wasn't because someone told him that Tenkyu realized it was love. Guest also never told Tenkyu that the feeling was love. Instead, they taught him emotions in other ways, hoping he would realize it himself.

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.