It's hard to pinpoint one individual. The genres have been shaped by numerous writers throughout history. Some early pioneers like Jules Verne and H. G. Wells played significant roles, but countless others have added to and expanded the scope of science fiction and fantasy.
There isn't a single person who can be credited as the sole creator. Science fiction and fantasy have evolved over time with contributions from many authors and storytellers.
The creation of science fiction and fantasy is a collective effort. Many authors have contributed to their development. From Mary Shelley to modern-day writers, each has added their unique ideas and imagination to these genres.