Evil Shiro Tokisada Amakusa (天草 四郎 時貞 (悪), Amakusa Shirō Tokisada (Aku)) is the evil side of Shiro Tokisada Amakusa. He is alternately referred to as "the dark half of Amakusa's soul." (闇側の魂, yamikawa no tamashii). He is the boss and main antagonist in Samurai Shodown IV. His purple skin indicates the malevolence of his ki energy.
On his second revival, his spirit was split into two halves, one good and just; the other corrupt and ambitious. The evil half brings destruction to Japan from Shimbara, and revives his efforts for revenge by stealing Zankuro as an implement for destruction. He kidnaps a young kunoichi named Hazuki from the Kazama ninja clan. Hazuki, adapt with ki and wielding a special life force inside her, is used by the sorcerer to control Zankuro's bloodlust, and also to keep the existence of his castle.  According to the official story, he was slain by Kazuki.  After his defeat Hazuki is released and his castle crumbles back into nothingness.
A being who wants nothing more than to use his powers for destruction and unfathomably pure evil. This evil side of Amakusa demands total subjugation and like fellow villain M. Bison possesses characteristics of a psychopath.
- Levitation - Amakusa has the power to levitate his weapon and other characters off the ground.
- Teleport - Amakusa can teleport himself.
- Multiple Attacks: Amakusa can deliver several slaps at once.
- Fire Projectile: Amakusa has the ability to project a fiery image of a demon.
His mode (Shura/Slash or Rasetsu/Bust) always opposes the mode of player character. Amakusa fights his opponents by attacking them with a levitating gem. He can direct the gem with an impressive range, able to smash his enemies from the air and ground level from nearly any distance easily. At times, his gem may not compensate for opponents close to him so he also has a multi-hitting slap attack when he is close. To confuse his adversaries, he may also teleport and hover above them before diving upon them with a damaging attack. He also projects fiery projectiles in the image of demons to his foes, making him a formidable long-distance fighter.
- Movement to Tranquility - Samurai Shodown IV
- Eiji Yano - Using doubled voice filter to emphasize the evil nature of the character.
- SNK Playmore. "Samurai Spirits Genealogy". Samurai Shodown V.