Final answer:
William Afton possesses the character Springtrap in the Five Nights at Freddy's series, becoming the main antagonist from the third game onwards.
Step-by-step explanation:
The character that William Afton possesses in the Five Nights at Freddy's series is B)Springtrap. William Afton, also known as the Purple Guy, becomes trapped inside the Springtrap animatronic after trying to hide inside the Spring Bonnie suit, which malfunctions and crushes him. He later becomes the main antagonist in the series, starting from Five Nights at Freddy's 3, where he is revealed to be inside the Springtrap suit.