Rating issues
Only when you shoot an opponent will his body become visible, thus the violence is implied and not actually shown on screen. This body is subjected to the physics of the game and falls in a rag doll fashion to the ground.
The characters you control are assigned randomly (and sometimes vary during one play session). They always have a humanoid appearance, but are not always human. Sometimes you’re playing as a man-chicken, a skeleton, fish head with legs, or a human with a big old diving helmet on. The characters aren’t textured and consist of one solid colour, making them even less ‘real’.
Other issues
The game features an extensive list of modifiers (options that alter the gameplay a bit). For instance, there is a low gravity setting, baby mode (small character), etc. One of these modifiers is called ‘show characters’. When this mode is selected, nobody is invisible anymore. Important fact; the characters can be seen, but aren’t animated! You see a floating gun with a ragdoll character attached to it. When shot, the character falls to the floor like during regular gameplay.