Clowns

The Clowns (ピエロ, piero, from French pierrot) are a group of ghouls who seem to have triggered and observed the events of the series. They possess clown masks.

Members

 * Uta
 * Itori
 * Nico
 * Roma Hoito
 * Souta

Involvement
There are subtle hints dropped around Rize's death, a stunt orchestrated by the clowns, resulted in Kaneki being turned into a ghoul and led to his protagonist's loss of humanity, and many other cases of ghoulification involving ghouls and humans. The following incidences were caused by the clowns, either directly or indirectly:

Rize's death, a stunt orchestrated by the clowns, resulted in Kaneki being turned into a ghoul and led to his involvement with Anteiku. related to the Yamori incident in the Aogiri arc. Prior to the events of the series, the clowns handed Yamori over to the CCG investigators in the 23rd ward due to his knowledge of the happenings in the 13th ward. Yamori suffered through brutal torture and emerged with a sadistic personality, using it to his advantage on Kaneki. and caused him to meet Tsukiyama, where he was almost eaten by the hands of its customers.

Nico gave Kaneki information that resulted in him finding Kanou's lab and seeing Rize briefly. Up to this point, Kaneki believed that all of his strength came from Rize. As a result, seeing her broken made him question himself and his strength, increasing his mental distress.

CCG became more vicious as Uta told Kaneki about The Reaper, mentioning that he was horrible and someone to be feared. Kaneki was stunned by Arima's strength upon meeting him, leading to his quick defeat due to the impairment of his mental state.

Trivia

 * Coincidentally, the animation studio working on the Tokyo Ghoul anime is also named Pierrot.