In anime, or manga, they are characters who begin the series as an antagonist. A mayadere
will fall in
love with the main character, but that doesn't mean they
will switch sides, and are willing the kill their love interest if they feel betrayed.
An example of this is a specific character in Akame Ga Kill, her name is Esdeath, and you should watch that
anime.
Weeb1: "NE NE, Watashi Wa Think That One Waifu From (insert-
anime-that-has-{loli}), Is Kawaii..."
Weeb2: "Nani? Omae Wa Gay~desu, Watashi Only Go For Evil Waifus..."
Weeb1: "Anata Wa Mean A Mayadere?"
(TRANSLATION)
Degenerate1: "Hey Hey, I Think That That One (false-2d-girl) Waifu From (insert-
anime-that-has-(an-underaged-girl)), Is
Cute..."
Degenerate2: "What? You're Gay, I Only Go For Evil (false-2d-girls) Waifus..."
Degenerate1: "Do You Mean A Mayadere?"