Enmu stands as the central antagonist within the Mugen Train narrative. His human origins trace back to a lifelong inability to separate dreams from reality, fostering troubling conduct. As an adult, he adopted the guise of a medical practitioner, manipulating patients through hypnotic therapies and falsely assuring terminally ill individuals of cures. He continued negligent treatments despite awareness of his own insincerity.

A fatal meeting with Muzan Kibutsuji altered his existence. As Muzan consumed his intestines, Enmu expressed admiration for Muzan's lack of pain. This reaction spurred Muzan to whimsically transform him into a demon. Knowing his transformation was arbitrary, Enmu dedicated himself to climbing the demon hierarchy, ultimately attaining the rank of Lower Rank One among the Twelve Kizuki.

Physically, Enmu presents as a slender young man of average height and medium build, with pale skin. His features include downward-tilting blue eyes; the left iris bears the kanji for "Lower One," while the right contains a horizontal slit. Three square markings under each eye shift from green to yellow. His black hair displays pink and blue gradients at the tips, with two longer strands tinted green. After receiving more blood from Muzan, a distinct mouth and eye manifested on the back of his left hand, and the kanji for "Dream" appeared across most fingers.

His personality reveals coldness, callousness, and pronounced sadism. He openly relishes human suffering, particularly by luring victims into pleasant dreams before twisting them into nightmares to savor expressions of agony. This sadism extends to fellow demons, evidenced by his pleasure during Muzan's execution of other Lower Ranks. He also displays masochistic tendencies, experiencing euphoria during the excruciating process of receiving Muzan's blood. He views humans as expendable pawns, manipulating them with false promises of eternal peaceful sleep in exchange for service. Typically cautious and strategic in exploiting his abilities' strengths and weaknesses, he grows overconfident and arrogant after gaining more of Muzan's blood, recklessly seeking to challenge an Upper Rank.

His abilities encompass the enhanced speed, reflexes, and flesh manipulation characteristic of high-ranking demons. He can detach body parts like his sentient hand for independent tasks and create flesh puppets as decoys. His Blood Demon Art focuses on sleep induction and dream manipulation:
- Sleep can be induced via blood-infused train tickets, a whispering mouth on his hand, or eye contact with numerous "Dream"-kanji eyes after fusing with the train.
- Dream manipulation allows entry into subconscious realms to locate and destroy a victim's "spiritual core," crippling the mind.
- Special ropes enable others to enter dreams, while bone awls pierce spiritual cores.
His most significant feat involves fusing his body with an entire train to devour all passengers simultaneously.

During the Mugen Train events, he executed an intricate plan:
1. Deploying human pawns suffering from insomnia to enter Demon Slayers' dreams and destroy their spiritual cores.
2. Repeatedly forcing Tanjiro Kamado into sleep when confronted.
3. Revealing his fusion with the train after suffering temporary decapitation.
His strategy ultimately failed due to the Demon Slayers awakening and coordinating their efforts. Inosuke Hashibira's resistance to hypnosis aided in locating Enmu's neck within the train structure, leading Tanjiro to sever it using Hinokami Kagura. This caused a derailment and Enmu's disintegration.

His final moments show a refusal to accept responsibility for his defeat, instead cursing the Demon Slayers, Nezuko, and the Upper Ranks while lamenting his perceived inferiority. He attempted to crawl toward the weakened Tanjiro and Inosuke but crumbled before reaching them.