Fuyuki Shido, also known by the title of Beastmaster, is a prominent character who stands at 182 centimeters tall and has a birthday of August 12. His background is deeply tied to the Infinity Fortress, a vast and lawless city within the series' world. Shido was once one of the Four Kings of the VOLTS, a powerful gang within that fortress led by Ginji Amano. When Ginji decided to leave the Fortress to live on the surface, Shido felt a strong sense of betrayal and abandonment, which led him to depart as well, seeking to make his own way in the outside world. For a time, he worked for a man named Atsushi Akutsu before eventually reconciling with Ginji during a mission to recover the Sound of Life. Following this reconciliation, Shido took inspiration from the Getbackers' retrieval business and started his own similar service.

In terms of personality, Shido is generally portrayed as independent, serious, and often appears cold or aloof towards others. Despite this exterior, he holds a deep respect for Ginji as a friend and possesses a strong moral code, particularly concerning his unique abilities. He views animals not as tools but as friends and allies, preferring to ask for their assistance rather than exert control over them. This perspective highlights a protective and empathetic side to his character. One of his most notable personality traits is his ongoing rivalry with Ban Mido, which manifests in constant bickering; Shido frequently refers to Ban as snake bastard or snake-for-brains, while Ban retaliates by calling him monkey boy or pimp.

Shido is a descendant of the Maryuudo, an ancient clan with the ability to communicate with animals and take on their characteristics. His proficiency with this power is exceptional, to the point where he is described as a genius who appears only once every hundred years, granting him the ability to mimic roughly one hundred different types of animals. This technique, known as Hyakujuu Gitai or Beast Mimicry, allows him to transform parts of his body to gain specific animal traits. For example, he can use Bat Form for echolocation, Wolf Form for powerful claws and fangs, Primate Form for enhanced jumping and agility, and Grizzly Form for immense strength. He can also summon animals to assist him in various tasks.

Beyond his Beast Mimicry, Shido holds a far more significant power as the last survivor of the Fuyuki Clan. He possesses a form known as Chimera, which is one of the three keys required to open the sealed gates of Babylon City. When this power is activated, his physical capabilities become immense. With a simple flick of his arm, he can level a building, and his endurance and willpower are amplified to supernatural degrees, effectively making him immune to attacks he believes cannot harm him. Shido also possesses a Shikizoku power called Awakening, which allows him to release the latent potential hidden within a person. He notably used this ability to restore Ginji Amano's full powers.

Throughout the story, Shido's motivations are driven by loyalty and a desire to protect. He initially follows Ginji out of respect but must reconcile his feelings of betrayal. His primary role often involves confronting physical threats and providing formidable support in battle. A key relationship in his life is with Madoka Otowa, a violin prodigy who becomes his girlfriend and with whom he eventually lives, representing a significant emotional development for the otherwise aloof character. Another important relationship is with Masaki Kurusu, having been paired with him to protect the East Gates of the Infinity Fortress during their VOLTS days. This history gives Shido deep insight into Kurusu's character, allowing him to note how much he has changed from being a man who once protected the weak.