The mouse is a ghost role that you can select upon death, or if you join as an observer.
The mouse must feed! Avoid getting killed the best you can. You're just a little guy, though! Be careful of bloodthirsty passengers, protective chefs, and psychopathic security officers. All of these are your natural predators.
The mouse scurries under locked doors to get access to their precious, precious food. If it has been electrified by a station engineer, you will die when attempting this. Be careful out there!
The mouse can eat with Alt+click
Remember that once you have assumed the role of a mouse, you know nothing of your former life. All friendships are dissolved, old hatreds forgotten. New ones will take their place, don't worry.