Evil twin usually refers to a person with whom someone shares a mischievous, troublemaking, or rebellious dynamic. Despite the name, evil twins aren't necessarily related, and they may or may not be involved sexually, romantically, or in a dynamic. The relationship is often described as sibling-like rather than romantic or sexual.
Origins of the Term
Evil twin is an old phrase (from around 1910-1915), not a term that started in kink. It originally referred to the classic fictional idea of a good person having an identical but bad or troublesome twin.
By the early 2000s, people were also using evil twin jokingly online for a close friend who shared their mischievous side. The kink use of this term likely grew out of this playful internet meaning.
