Female characters in anime and manga who are, were, and always will be wicked and evil.
Not all akujo are female antagonists, nor are all female antagonists akujo. Akujo is the Japanese word for females who are genuinely evil. In general, this means that the character remains a bad/wicked individual during her entire run in the series, including any available flashbacks or prequels that delve deeper into her background.
A true akujo is not a victim. Her motives do not stem from a dark period in her past, such as traumatic abuse or getting revenge for a wrongdoing committed against either her or someone she cares deeply about. They also never ally themselves with the good guys (aside from an intent to deceive), or make any other reformed change that would make them seem redeemable or sympathetic in the eyes of the viewing audience. Naturally, akujo also commit their heinous acts of their own free will; never under the influence of mind control or blackmail.
In contrast, an akujo often has a sadistic streak; taking enjoyment in causing pain to the innocent or virtuous, and having very little qualms about attempting or succeeding to kill in cold blood. Self-indulgent betrayal and malevolently-motivated manipulation are also strong characteristics.