Diego is a master wax modeler residing on Hand Island, where he is known as a skilled craftsman. He is the father of Regis, a young man who joined the Marines. Diego is a large, tanned man with a broad and muscular upper body that contrasts sharply with his disproportionately narrow legs. He has white hair that extends down his neck and white sideburns, usually covered by a red cap. His attire consists of blue overalls, a string of blue beads around his neck, a white tool belt around his waist, and a brown cloak draped over his shoulders. Due to his advanced age and the physically demanding nature of his work, he often suffers from backaches.

In personality, Diego is a stubborn craftsman who takes immense pride in his wax modeling skills. He is also deeply proud of his son, Regis, who inherited his artistic talent. Diego is a peaceful and emotional man who cherishes freedom above all else. Throughout his long life, he has encountered numerous infamous pirates, including Silvers Rayleigh, Edward Newgate, Shanks, and many members of their crews. These encounters led him to greatly admire pirates for their free-spirited way of life, and he developed a habit of creating wax statues of those he met. His admiration for this lifestyle shaped the principles with which he raised his son.

The central relationship in Diego's life is with his son, Regis. He trained Regis in the art of wax modeling and was overjoyed when his son created his first figure at the age of six. Their relationship became strained, however, after the death of Whitebeard, whose protection had kept Hand Island safe. With the island's security compromised, a Marine commander named Bilić took control, and Regis chose to join the Marines against his father's wishes. This decision led to a painful separation between father and son. Despite the conflict, Diego's love for Regis never wavers, and he keeps the first wax figure his son ever made as a treasured memento. Diego also develops a strong respect for the Marine officer Koby, recognizing him as a good person who admires the pirate Monkey D. Luffy, despite their opposing allegiances.

Diego's role in the story begins when the Straw Hat pirates crash-land on Hand Island, destroying his home. He meets Monkey D. Luffy, and the two bond over their shared interest in pirates. Diego shows Luffy his extensive collection of wax statues in his basement, which includes figures of many legendary pirates he met in his youth. When Diego decides he must deliver the first wax figure made by his son to Regis, his back problems prevent him from making the journey, so Luffy offers to go in his stead. Later, Diego uses his artistic talents to help end Bilić's tyranny. At Koby's request, he creates a highly realistic wax replica of Admiral Kizaru and uses his ability to perfectly imitate the Admiral's voice. This trick fools the Marine soldiers and allows them to arrest the corrupt Bilić. In the aftermath, Diego reconciles with his son, finally accepting Regis's decision to follow his own path as a Marine. He then begins work on a new wax statue, this one made in honor of Monkey D. Luffy.

Diego's notable abilities are his exceptional skills as a wax modeler. He is able to create incredibly lifelike and durable wax statues of any person, with his replica of Admiral Kizaru being so convincing that it fooled a base full of Marines. In addition to his sculpting talent, he possesses a remarkable gift for voice impersonation, being able to mimic the distinctive voice of Admiral Borsalino perfectly. His wax is also shown to be extremely hard, as a statue can be made strong enough to withstand the pressure of a cannon firing. Despite his physical limitations due to age and back pain, he remains determined and resourceful, proving that his craft is a formidable skill in its own right.