Stone River, alias Souseki Ishinagare, operates as a core antagonist within the franchise. He belongs to the Three Cards, a group serving Lady Arsene of the Phantom Thief Empire. Maintaining a covert identity as head chef and groundskeeper at Holmes Detective Academy, he executes duties for the empire while overseeing school maintenance.

His personality blends unwavering loyalty and cold efficiency toward Lady Arsene’s objectives, contrasting with his undercover persona. As Souseki Ishinagare, he demonstrates unexpected diligence and care for the academy’s students, performing tasks like cooking, cleaning, laundry, and gardening. Despite this facade, his devotion to thievery and Arsene remains absolute. He exhibits formidable combat prowess, particularly swordsmanship, and excels in practical endeavors.

His primary ability, Dollification, stems from his Toy. Through eye contact, he immobilizes targets like statues, with effects typically fading over time. A notable instance saw him petrified by his own ability after glimpsing his reflection on a plate held by Cordelia at the first anime season’s conclusion.

Series developments reveal occasional softening of his stern demeanor due to prolonged undercover exposure, though his loyalty to Arsene persists unshaken. During the second anime season, he coordinates attacks with fellow thief Twenty, employing named combo techniques like "Choukyuu Haou Den'eidan" and "Sekiha Love Love Tenkyoken," which nod to other media. The destruction of Milky Holmes’ garden provoked intense backlash from protagonists toward him and other thieves.

His design includes a November 4 birthday, 176 cm height, 63 kg weight, and blood type B. His character name draws inspiration from historical figure Ishikawa Goemon.