Mr. Cateaux

Mr. Cateaux is a character that appeared in Chapter "X-Day Protocol" of the Steins;Gate 0 visual novel and in Chapter 27 of the manga adaptation.

Steins;Gate 0 Visual Novel/Manga
Mr. Cateaux participated at the ATF seminar held by Maho Hiyajo and Alexis Leskinen. After he criticized the Amadeus System (and indirectly Kurisu), Okabe publicly rebuked him.

X-Day Protocol
Okabe, Maho and Leskinen meet Professor Izaki at the parking garage but before they can leave, the car windows shatter and they suddenly come under attack by an assailant, Mr. Cateaux. Okabe and the others all take cover behind the car, and Okabe looks over and sees the assailant, who seems vaguely familiar to Okabe and is holding something like a makeshift handgun. He then hears the assailant mumble something about cutting off that which causes you to stumble as it's better to end life maimed than be subject to eternal suffering, and then saying something about how a soul comes from God and can never be placed in something artificial. Cateaux then fires his weapon again, but misses as Professor Izaki runs away. Okabe then gets everyone in the car and tries to drive forward, but is not able to since the clutch is in neutral. Cateaux then shoots at Okabe, and Okabe and Maho make an attempt at escaping with a car. They haphazardly drive forward, almost hitting the assailant, and carelessly race through the parking garage for a while, until they spin out. As they try to switch drivers, they notice another car heading straight for them with the assailant as the driver. They quickly get out of the way right before Cateaux drives into their car at full speed, seemingly killing himself in the process.

However, Mr. Cateaux, while grieviously injured, remains alive. Pulling himself out of the wreckage of his car, he attempts to shoot Okabe, Maho and Leskinen; however, he is suddenly shot dead by an unknown assailant.

Trivia

 * The man Okabe criticised at the ATF seminar in the anime is a different person and the parking sparatory scene from X-Day Protocol is entirely absent.