Two characters who seem very comfortable with each other to the point that it seems like they have been married for a long time, though obviously it doesn't apply to couples who actually have been together for a long time. Usually it is done through constant arguments with each other, as only people who are so closely bonded can have such open communication between them (whether they will admit it or not). Someone is apt to comment that they behave just like a married couple.
Typically the argument will be a back and forth of opinions, such as a "Cavemen vs. Astronauts" Debate. It is likely to get heated but neither of them are willing to just leave. In extreme cases it might start getting ugly with insults about personal issues being flung about. If not an argument based, it is often tied with Heterosexual Life-Partners where domestic issues are a common topic. The two bickering characters in question do not have to be romantically involved with each other as the trope can apply to any two individuals such as best friends, siblings, military comrades, and a couple of strangers who don't know much about each other and are not romantically or sexually involved but their situations make them bicker as if they knew each other for a long time
