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?"