Esu

Esu, otherwise known as S, is a four volume Boys Love Genre Light Novel 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."

Tropes used in Esu include:

Bishonen: Shiba is repeatedly called pretty
Dead Little Sister: Andou; Shiba has a Dead Older Sister
Even the Guys Want Him: Shiba
Heroic Bastard: Munechika.
Idiot Ball: Shiba gets this in the first volume when 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.
Love Dodecahedron: Shiba with Andou, Shinozuka, and Munechika. Although some of the connections are more of an Obsession Dodecahedron.
Shown Their Work: The series contains much detailed information about the police force, and the mafia.
Pants-Positive Safety: Shiba, on the cover of Volume 1 and a few of the illustrations, although not in the story proper.
Poor Communication Kills: "Watch out for your S" does not mean that your S will betray you.
Unrequited Love: Andou for Shiba.
We Hardly Knew Ye: Andou
Yakuza