The pithy answer is: They’re best friends that like fighting each other, but sometimes the armor comes off, and one of them has a collar and there’s dicks and Lantresor is in your mine, who knows?
The actual answer is they’re sparing partners and pretty intense competitors; they’re always trying to outdo one another. Infrequently there’s dicks. They’re not in any sort of romantic relationship; they have romantic relationships with others.