Keika Yō is the central protagonist of Spiritpact, a young man who serves as the heir to an old and once-famous family of exorcists. However, following the death of his parents, his life falls into poverty, a stark contrast to his family's illustrious past. To make a meager living, he works part-time as a street fortune teller and a computer repair technician. It is during one of his late-night scavenging trips to a junkyard that his fate is sealed; he encounters the exorcist Tanmoku Ki fighting an evil spirit and, shortly after this meeting, dies in an accident, becoming a ghost.

In the initial stages of the story, Keika is portrayed as a carefree but emotionally volatile character. Early impressions highlight him as rude and obnoxious, with a tendency to whine and react with disbelief to the supernatural world he is thrust into. Despite this abrasive exterior, his personality evolves as he grapples with his new existence. Given a choice between fading away or becoming a "spirit shadow" to Tanmoku Ki, he chooses to form the pact, binding his soul to the exorcist. Initially weak and unable to fight, Keika grows significantly over the course of the series, eventually earning the title of The Most Powerful Spirit Shadow.

His primary motivation shifts from mere survival to a deep-seated loyalty towards Ki. As he learns more about Ki’s burdens and the responsibilities of an exorcist, he resolves to protect Ki at all costs. Their bond is the emotional core of the narrative; they are tethered as light and shadow, with their connection essential for maintaining the world's balance. A key relationship that defines his role is with his sword, Rakugetsu. As a direct descendant of You Nei, Keika is one of the few who can wield this powerful weapon, which connects him to his ancestral legacy. He also develops a complex relationship with Ritsu Shito, the son of the Tanmoku family's butler, who harbors a deep hatred for Ki.

Throughout the series, Keika undergoes significant development as he uncovers the mysteries of his own bloodline and the secrets of the Tanmoku family. He proves that his worth is not just as a shadow, but as a powerful exorcist in his own right, directly confronting enemies like Sekijin Shinryuu and overcoming attempts at hypnosis and manipulation. His notable abilities are largely tied to his spiritual pact and his lineage. He can manifest weapons as spiritual tools, eventually mastering the legendary sword Rakugetsu. His strength is such that he becomes a formidable fighter, capable of making critical decisions in battle to save his partner, even if it means destroying the talisman that keeps his own soul tethered to the world.