Tribros believe in [[Team Free Will]]. As opposed to fans who are particularly attached to one brother or the other, or to only [[Sam]] and [[Dean]], or just [[Castiel]], etc. Some fans identify as "tribro," indicating that they are devoted to the relationship between all three. It arose after the eleventh season finale of ''Supernatural'' when Dean clearly stated that Castiel is a brother, by fans who wanted to be clear they had affection for all three brothers.
Even though Dean has said Castiel was like a brother to him before, in [[6.20 The Man Who Would Be King]], Dean said in [[11.23 Alpha and Omega]]: "You're our brother, Cas. I want you to know that."