Boogiepop Doesn't Laugh

Boogiepop wa Warawanai
ブギーポップは笑わない

Source

Boogiepop and Others
Boogiepop wa Warawanai· ブギーポップは笑わない
novel #74

There is an urban legend that children tell one another about a shinigami that can release people from the pain they may be suffering. This "Angel of Death" has a name—Boogiepop. And the legends are true. Boogiepop is real. (Source: Seven Seas Entertainment)

Prequel

Boogiepop at Dawn
Yoake no Boogiepop· 夜明けのブギーポップ
manga

The origin of Boogiepop revealed at last. Not only is this the long-awaited story of how Boogiepop was born, but it is also the tale of four very strange individuals: a detective, an eater of fear, a popular writer and a skilled assassin. Uncover all the secrets.

Sequel

Boogiepop and Others VS Imaginator
Boogiepop wa Warawanai VS Imaginator· ブギーポップは笑わない VSイマジネーター
manga

Witness the first appearances of the mysterious Towa Organization - a player in the Boogiepop Phantom anime series - as Boogiepop finds himself hunted by the synthetic human known only as "Spooky E." All the while, the deadly "Imaginator" is out to change the world one mind at a time. Can you escape from the clutches of the Imaginator...?

Alternative

Boogiepop Dual
Boogiepop Dual: Makeinu-tachi no Circus· ブギーポップ・デュアル 負け犬たちのサーカス
manga

When a female student is abducted and on the verge of being raped, Boogiepop rushes onto the scene just in time to save the day. But Boogiepop's new male alter-ego, Akimizuki Takaya, finds himself in the middle of a crime wave with ties going back to the previous owner of the Boogiepop mantle. (Source: Wikipedia)