The answer of the twelfth riddle - Hot Reunion Turned Cold

This one riddle is quite long, so I'll explain in each paragraph then...

The first paragraph.

When the sudden hot reunion turned cold

And the beautiful one was downed

Leaving the others to became paranoid.

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» It's about the reunion, and there's one woman in the group died. Others were paranoid, that meant it's probably murder.

The second paragraph.

There's no sleeping act this time

As the determination was on full-power

No one could get away when he's around

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» "Sleeping act", we all know it's Kogoro-san (but actually Conan-kun XD). He didn't need Conan's help because he's determined to find the culprit of this particular case.

The third paragraph.

Couldn't believed what he'd found out

Trying, assuring it wasn't the truth

Though that's not the case...

That made him finally released his pure anger

...He was betrayed...

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» Kogoro-san found out who was the killer. Apparently, he did not pleased with the outcome, that someone he didn't want to be culprit, actually was. He was furious, of course, who wouldn't when you're betrayed by someone you know? Now it's clearly Kogoro-san knows this person well and that he's probably the member of the reunion group.

The fourth paragraph.

Just who exactly, was the person

Who had ruined all the fun?

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» This asked who was the culprit of this case.

The Conclusion

When it's come to the certain reunion which Kogoro-san was involved, there's one case called "Kogoro's class reunion" case in episode 27 and 28. The members were Kogoro's old classmates from Beika University's Judo Club. The place of the case was in the hot spring's inn. The Beautiful one referred to Horikoshi Yumi-san, a teacher. It was at first looked like a suicide, but turned out it's murder instead.

Kogoro-san investigated the case with the great determination, so Conan-kun let him by decided not to use the stun gun wristwatch this time, and chose to give Kogoro-san clues so he can solve the case by himself.

Finally, Kogoro-san figured out who's the culprit : Nakamichi Kazushi. When he confessed and after Kogoro-san's sarcastic words, Kazushi grabbed his yukata, and got thrown by his Judo. He was sad afterward, but got spirited up when he saw girl's pink underwear. Pervert...

And from now on I'm going to use this style in the upcoming riddles, okay? XD