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''Esu'', otherwise known as ''S'', is a four volume BoysLoveGenre LightNovel series written by Saki Aida and illustrated by Chihara Nada. All four volumes have been published in America by Juné.

Shiba is a detective in the 5th section anti-organized-crime division of the Tokyo Metropolitan Police, a.k.a. "Anti-Gang 5", specializing in handgun seizures. Shiba's secret weapon is an "S" ("spy"), an informant who plays a key role in Shiba's information gathering. Shiba is training an informant who has been officially accepted into Shinjuku's Matsukura Group within the Budoha crime syndicate. One day, Shiba awakes to an anonymous phone call. "Watch out for your S."

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* {{Bishonen}}: Shiba is repeatedly called pretty
* EvenTheGuysWantHim: Shiba
* HeroicBastard: Munechika.
* IdiotBall: Shiba gets this in the first volume when [[spoiler: he receives a gift from a guy he was investigating that turned out to bugged. The guy finds out that Shiba is a detective and proceeds to drug him; this is after Shiba was repeatedly warned by Andou, Munechika, and his superiors not to continue meeting him.]]
* LoveDodecahedron: Shiba with Andou, Shinozuka, and Munechika. Although some of the connections are more of an Obsession Dodecahedron.
* ShownTheirWork: The series contains much detailed information about the police force, and the mafia.
* PantsPositiveSafety: Shiba, on the cover of Volume 1 and a few of the illustrations, although not in the story proper.
* PoorCommunicationKills: "Watch out for your S" does not mean that your S will betray you.
* UnrequitedLove: Andou for Shiba.
* WeHardlyKnewYe: [[spoiler: Andou]]
* {{Yakuza}}