A character with this tag has a haraguro personality.
A haraguro character is one who has an outward appearance of being amicable, friendly, innocent, kind and/or refreshing, but uses any combination of the aforementioned traits to hide their real cruel, cunning, evil, manipulative, mean and/or sadistic side.
Despite the negative connotation, not all haraguro are villainous or are sociopaths. Their motives vary very widely depending upon the character her/himself and the "seriousness" of the story.
The term comes from the Japanese word, "Haraguroi (腹黒い)", which means "black-hearted; mean; scheming".