Celty Sturluson is a central figure in the urban landscape of Ikebukuro, known to its inhabitants as the Black Rider or the Headless Rider. She is a Dullahan, a fairy creature from Irish folklore traditionally tasked with guiding souls to the afterlife. Her journey to Japan began over two decades before the events of the series when her head was stolen by the scientist Shingen Kishitani using a mystical blade. Severed from her head, she lost most of her long-term memories but retained a faint connection to her missing part, which she followed across the sea. Upon arriving in Tokyo, she entered into an arrangement with Shingen: he would provide her with a place to stay in exchange for being allowed to perform a full autopsy on her, an experience she found deeply traumatic but which solidified her unusual living situation with his son, Shinra Kishitani.

Despite her supernatural origins and fearsome reputation, Celty possesses one of the most grounded and relatable personalities in the series. She is generally confident, optimistic, and kind-hearted, often going out of her way to help strangers in distress, such as when she rescues a young girl attempting suicide, explaining that the world is not as cruel as it seems. This genuine nature is balanced by a dry sense of humor and a tendency to become flustered, especially in her interactions with Shinra, whom she often punches in the stomach when he makes declarations of his love for her. Her hobbies are decidedly human; she enjoys watching television and DVDs, playing video games, and chatting online under the alias Setton. However, she harbors some very specific and ironic fears, including a phobia of extraterrestrials and a growing unease toward traffic officers who frequently chase her through the city.

Her primary motivation for much of her time in Japan is the recovery of her stolen head. This quest is driven not only by a desire for completion but also by a deeper fear; she believes that if her head were to be destroyed, she would also cease to exist. The uncertainty of not knowing her own past and the vulnerability of having her fate tied to a missing piece of herself are significant sources of insecurity. Over the course of the story, especially into Durarara!!×2 Ten, this motivation evolves as she begins to build a life for herself. She comes to value her identity as the person she is in the present, a person who loves Shinra and has forged meaningful connections, rather than the being she might have been before losing her head.

Her role in the story is multifaceted. She works as an underground courier, transporting goods and information for various clients, a job that often brings her into contact with the city's most dangerous and eccentric figures, including the information broker Izaya Orihara, whom she cooperates with reluctantly despite not liking him. She becomes a trusted ally and protector to several of the main human characters, including Mikado Ryuugamine and Anri Sonohara, accepting their secrets as they accept hers. As a member of the anonymous online gang Dollars, she serves as a stabilizing force and a symbol of the city's hidden, extraordinary nature.

Her most significant relationship is with Shinra Kishitani, an underground physician who has lived with her for twenty years and has been in love with her since childhood. Their bond is complex and deep; Shinra loves her specifically as a headless Dullahan, which both comforts and frustrates her. She eventually confesses her love for him, solidifying their partnership. She also forms key relationships with other powerful figures in Ikebukuro, such as the incredibly strong but gentle bartender Shizuo Heiwajima, who accepts her without judgment, and the supernatural entities who wield the cursed blade Saika. Her interactions with the broader cast deepen her integration into the human world and challenge her perceptions of normalcy and monstrosity.

In terms of development, Celty’s journey is one of self-acceptance. She transitions from a creature singularly focused on reclaiming a lost part of herself to a person who actively chooses her present reality. While she continues to seek her head to control her own destiny, she becomes less defined by its absence and more by the life and relationships she has built. This growth allows her to confront the manipulations of characters like Izaya and to play a decisive role in the chaotic conflicts that engulf the city, moving from a passive seeker to an active agent in her own fate and the protector of her chosen home.

Celty possesses a range of notable abilities that stem from her nature as a Dullahan. Her most defining power is her control over a durable, shadow-like substance that constantly emanates from her body. She can shape this shadow into anything she desires, from her everyday black riding suit and helmet to complex tools, protective armor, and her signature weapon, a large, menacing scythe. Her physical strength is far above that of a normal human, and she has an accelerated healing factor that makes her nearly indestructible; she can feel pain but recovers from wounds rapidly. She does not have a beating heart or functioning organs in the human sense, and she perceives the world not through a head, but through the shadowy particles that act as sensory organs, granting her a slightly wider field of perception than a person. Her motorcycle, which she calls Shooter, is actually her supernatural horse, a Cóiste-Bodhar, disguised in the form of a black bike.