Kurumi Tokisaki is the main antagonist in the series. She is the third Spirit to be encountered in the series and is the most dangerous and lethal Spirit. She is heavily obsessed with Shidou and has the ability to manipulate time. Her codename is Nightmare. She has a split personality: that of a gentle, mysterious high school girl with a slow talking pattern, and that of a psychopathic killer. Kurumi is very aggressive in her actions which makes it harder for Shidou to capture her.

Kurumi can be described a person whose personality is hard to read, possessing a well-polished dislike of humanity that borders on mistrust, something she shared with many Spirits in their debut. She is a skilled actress, capable of putting on the façade of an innocent, curious young girl when she needs to, shown when she registered as a student at the Raizen High School, even speaking in a very polite manner; however, under that mask lies a psychotic woman, prone to fits of insane laughter that possesses no regard for human life, regarding them as a food source to restore her own time, and thus, a disposable commodity. There is a twisted justice behind her methods, however, as the people that she brutally murdered were would-be rapists and animal abusers.
In hindsight, Kurumi possesses some degree of innocence that her psychotic behavior have not entirely ruined; she has a soft spot for small animals and other creatures. She once slaughtered four humans not because she needed to replenish her time, but because they were trying to kill a kitten, this is the closest to a humanizing emotion that she has.

Mana reported that Kurumi is the most dangerous spirit known, who has personally killed over 10,000 people which does not include the spacequake's victims within that statistics. Kurumi used unstated and unexplained methods to personally transfer into Shidou Itsuka's high school and classroom so she can get closer to him. Her purposes and reasons in this transfer are simple; consume Shidou Itsuka, in doing so gain their memories, goals, fears and obsessions. In some non-canon stories, the clone Shido seduced shows some "independent" attitude, almost rebellious, and openly acts lovey-dovey with Shido, which causes troubles to the "main" Kurumi and even leads to confrontation with her.

Kurumi has the ability to manipulate the 4th dimension, mainly time, but her main power is associated with controlling shadows. Shadow is her primary power while Time is her secondary power in terms of usage. Combined with the power of time, she can create a clone with all her experiences, memories, abilities, etc. These clones however are shadows of her past selves who contain feelings the current Kurumi no longer possesses, Kurumi will kill these clones if they are swayed away from her goals.

These clones are less powerful than Kurumi herself as she has mentioned in the last chapter of volume 3 and in the epilogue of volume 4. She can form a single clone by shooting herself with Zafkiel or bring out all of her clones at once from a pool of shadows which she also uses to draw them all back in and all the time they've collected. She can also call her clones partially through small fields of shadows, most often to hold someone in place and prevent them from moving.

To activate any power she needs, she first needs to call her Angel (regardless of whether or not the Angel has been summoned already or not), and to position both of her guns just like a clock: The musket on 12 and the flintlock points at the number of the bullet. Next she needs to shoot the bullet at the target with the hand gun. In case she needs it for herself, she will be required to shoot it at her head.

She also has the ability to call and dismiss spacequakes at will, as shown in the series while she confronts Shidou on the school's rooftop.