Chiaki Hayasaka is a major secondary character in Shin Megami Tensei: Nocturne. She is a friend and classmate of both the protagonist and their mutual friend Isamu Nitta. At the start of the game, she meets with the pair with the intention of visiting their teacher Yuko Takao, who they believe is recovering in a hospital in Shinjuku. However, they quickly realize that something is amiss when they realize that the entire building is seemingly devoid of both staff and patients. As a hint of her personal habit of looking down on others, she encourages her friends to search the hospital, preferring to stay behind in the lobby in order to read an occult magazine she takes from the protagonist. An article she reads in it foreshadows events that are about to transpire.
Like her friends, Chiaki is spared from the Conception by virtue of the fact that she was inside the hospital as it occurred. When she wanders outside into the Vortex World, she becomes separated from the others; lost, confused, and frightened for her life. Unwilling to accept help for fear of being perceived as weak, she slogs her way through the world as a festering hatred of her situation grows inside of her, eventually forming into what would become the seed of her Reason, Yosuga. With the philosophy Yosuga, Chiaki intends to create her ideal world; one in which she cannot be hurt and in which she is free to pass judgement on others. It would effectively be a world based on the concept of the survival of the fittest. In her ideal, Chiaki would be the most powerful, and thus the fittest of all. However, a world based in Yosuga would be a world of unending strife, and in reality, the one with the most power would be continually challenged by others seeking to take that power for themselves. To pursue Chiaki's goals would be to support a world of chaos.
After acquiring the determination to make her Reason a reality and build her world, Chiaki attempts to court the protagonist, now the Demi-Fiend, to her cause. Following this, she seeks a vast resource of Magatsuhi, energy required for the formulation of a reason, that is being stored in Yoyogi Park. However, she loses her right arm in a fight with the Manikin collecting the energy. After she retreats to the remains of Mantra Headquarters, she meets the spirit of Gozu-Tennoh. As the two realize that they would mutually benefit from joining forces, Gozu-Tennoh grants her his power. With this new strength, Chiaki takes control of Mantra, establishing the foothold of her Reason. She leads her forces in an effort to hunt down the Manakins and claim the Magatsuhi she seeks.
When Chiaki's demon sponsor grants her power, she is transformed into the Baal Avatar, and no matter what ending the player pursues, the Demi-Fiend must defeat her in the Tower of Kagutsuchi near the end of the game. Additionally, if the Demi-Fiend is allied with Isamu and his Reason of Musubi, she will hunt down Noah, and by extension Isamu, and kill them. Should the player choose to pursue the path of Yosuga to the end, Chiaki's spirit will speak to the protagonist as her ideal world is about to be created; she tells him the world of Yosuga can have only one ruler, and because of that, she's glad that it was he that defeated her.
Chiaki is unique amongst the three Reason creators for being the only one that is at any point explicitly referred to by others as cruel or evil. Her character arc takes her from the position of a stuck-up high school student to a merciless killer of the weak, whereas Isamu and Hikawa only fight those that pose a threat to themselves and their Reasons. This is even despite the fact that it was because of Hikawa that that the world was destroyed.
In the original Japanese version of Nocturne, Chiaki's full name is Chiaki Tachibana. Her surname was changed to Hayasaka for its western release due to the fact that when spelled using English alphabet characters, "Tachibana" is too long to fit into the game's character name field.