The Red Gun takes appearance on a large black gun. It's handle, bullet pipes and design are neon-red. When used, it fires a red laser that zaps a person, causing them to disintegrate.
The Red Gun appears for the first time (chronologically) in Part 2 of the SCHOOL TRAGEDY series after Kendall's minions, Megan and Bethany, kill Meatball. The witnesses, Jay and Markus, got interrupted by the gun randomly spawning on their table. It appeared again in next part, revealing that after Meatball's death, Markus obtained it. Jay, after witnessing him being shot by Beth and Megan, got surprised by the gun too like Markus.
Gun's effects were not shown until Gonna be fine (they were shown by production order first). Kendall orders her minions to shoot Chloe and Jay, but he manages to pull out the gun and shoot the trio, avenging Markus' death.