Who he is
Fu Xi (伏羲), also called Bao Xi, is one of the mythical rulers at the dawn of Chinese civilization. Legends credit him with teaching people to hunt with nets, to fish and to raise animals. Han dynasty reliefs show him with a serpent's body, intertwined with Nüwa.
The trigrams
The Great Commentary, one of the Ten Wings, tells how Fu Xi looked up at the patterns in the sky and down at the shapes of the earth. He studied the markings of birds and beasts, what was near in his own body and what was far in things, and then drew the eight trigram figures. It's a story about where the book comes from: close attention to the world.
His name in the I Ching
Song dynasty thinkers, Shao Yong above all, gave his name to an arrangement of the trigrams, the Earlier Heaven, and to an order of the 64 hexagrams that later caught Leibniz's eye for its likeness to binary numbers.
What history says
Fu Xi is a legendary figure, not a historical one. The earliest traces of the system are sets of numbers carved on Shang and early Zhou bones and bronzes, which some scholars read as forerunners of the hexagrams. Some even think the trigrams were worked out from the hexagrams, not the other way around.
With King Wen and Confucius, he makes up tradition's trio of sages.