Steve Ditko, the comic book artist who co-created Marvel’s Spider-Man and Doctor Strange, was found dead in his New York City apartment on June 29th, according to The Hollywood Reporter. He was 90 years old.

In a statement, the president of Marvel Entertainment, Dan Buckely, said that Ditko “transformed the industry and the Marvel Universe, and his legacy will never be forgotten,” while the company’s Chief Creative Officer, Joe Quesada said that the artist “blessed us with gorgeous art, fantastical stories, heroic characters and a mystical persona worthy of some of his greatest creations.” Other writers and artists, from Neil Gaiman to Mike Mignola to Alex Ross have paid their own tributes online.

Born in 1927 in Pennsylvania, Ditko grew up during the Great Depression. According to Blake Bell in his biography of the artist, Strange and Stranger: The World of Steve Ditko, those circumstances prompted Ditko’s working-class family to “make due with inexpensive forms of pop culture,” including newspaper comic strips. Ditko picked up his love of comics from his father, an interest that only deepened with the publication Bill Finger and Bob Kane’s Batman and Will Eisner’s The Spirit comics in the 1940s, which inspired him to pursue a career as a comic book artist. Following the end of World War II, he enlisted in the US Army and drew comics for the Army’s newspaper in Germany.

Following his discharge from the Army, he attended The Cartoonists & Illustrators School in New York City, where he studied under Batman artist Jerry Robinson, and eventually sold his first comic in 1953. For the next couple of years, he developed his style drawing horror comics for small publishers, before he was eventually spotted and hired by Stan Lee of Marvel Comics, where he wrote for titles such as Marvel Tales and Worlds of Suspense. The pair eventually began working closely together, with Lee writing and providing ideas, and Ditko illustrating them. According to Bradford W. Wright in his history Comic Book Nation: The Transformation of Youth Culture in America, their partnership “seemed to instill Stan Lee with new enthusiasm. After years of derivative genre hackwork, Marvel’s comic books finally assumed distinctive qualities.”

Their most famous collaboration became Marvel’s biggest hit: Spider-Man. While Lee had tasked fellow artist Jack Kirby with the art, Dikto soon took over: Wright says that Lee “appreciated Ditko’s ability to emphasize the weakness and fragility of his characters,” and that the artist “effectively brought out the humanity of the teenage hero.” Ditko redrew the character, creating his iconic red-and-blue costume. When the character first appeared in Amazing Fantasy #15 in August 1962, it was an unexpected hit, and defined Marvel’s approach to superheroes.

Ditko continued to draw for a dedicated comic line for the character, The Amazing Spider-Man, and helped create some of Peter Parker’s iconic villains, such as Doctor Octopus, the Sandman, and Green Goblin. He would go on to develop another famous Marvel hero, Doctor Strange, through which he was able to put to use some of his more surrealistic styling to work. Ditko abruptly left Marvel after he and Lee had a falling out in 1966, and he ended up working for DC Comics for a short while, before freelancing for a number of independent publishers (as well as freelance work for Marvel and DC), becoming heavily influenced by the Objectivist works of Ayn Rand. He was known as a reclusive figure, who rarely provided interviews, sign autographs, or appeared in public.

In recent years, superheroes have taken over the Hollywood box office, led in part by Ditko’s creations: Spider-Man, which has seen three different incarnations in recent years, while Doctor Strange, received its own big-budget treatment. Doctor Strange director Scott Derrickson noted that he never contacted the artist, telling The Hollywood Reporter that Ditko is “like J.D. Salinger. He is private and has intentionally stayed out of the spotlight.” Ditko retired in 1998, but continued to work out of his Midtown apartment.

With his death, Ditko leaves behind an incredible and complicated legacy. Far from the showman that is Stan Lee, he quietly devoted himself to his art and stories, creating some of the industry’s best-known characters.