The X-characters are evil guardian characters controlled by king secret revealer. When people do something wrong and punish their own hearts, the heart's egg transforms into an X-egg, which if allowed to hatch becomes an X-character. In one of the early episodes a small boy punishes his own heart, because he thinks Amu-chan looks down on him as an unworthy and stupid person. His heart's egg becomes an X-egg, but Amu-chan destroys it and it returns to its origanal state as a hearts egg. X-characters are highly unlike the other guardian characters, with their whole body black, no hair, hardly human, purple evil eyes with no white circle or pupil, and a definable bold red X on their forehead(with exception of X-Dia), giving them the title of X-characters and X-eggs, which are black eggs with a big white bold X. The episode which the above text describes is "Shoot! Get that X-Character!"
X-Characters can have eccentric and sometimes humorous personalities. They can even act like their owners at times, as Saya's X-Character demonstrated when Pearl's Guardian Character complimented its singing.
They often say words like "impossible" or "useless" when translated into english.