As twins, Ruffnut and Tuffnut are rarely ever seen apart. From the start, both have a habit of fighting one another from disputes on possessions to misunderstandings to simple boredom. They seem to operate on sadomasochistic tendencies, inflicting pain on one another (most commonly Ruffnut inflicting it on Ruffnut) and putting themselves into harm's way in creative ways. They both share Barf and Belch as their dragon, and both are needed in order to properly operate the zippleback.
However their relationship happens have been interpreted by fans all over, from tending to each other's injuries to exchanging ideas. For the sake of it, fans sometimes interpret them as secret lovers, sharing the name "twincest" with the concept of fanbases that pair twins together with each other.
Note: Please add only ones that are part of the RotBTD fandom