Billy Gates is a former godson of Cosmo and Wanda. He is based on the famous real-life creator computer mogul and creator of Microsoft, Bill Gates.
Billy Gates is a nerdy looking boy who was the former godchild of Cosmo and Wanda in the 1970. A time traveling Timmy Turner ran into Billy Gates.
When he appears as a child in 1970s Dimmsdale, he has a brown afro, thick coke bottle glasses, and a teal blue shirt. He speaks with a nasally voice like Elmer, which is not odd, because Billy shares the same voice actor as Elmer.
Billy Gates has appeared twice in the show. In "Father Time!", his first appearance, he is shown returning from traveling in time with younger Cosmo and Wanda from the 1940s just in time when Timmy was arriving in 1970. In this episode, after Cosmo and Wanda showing a computer of downloading lava lamps, he devised the idea of creating Internet. Cosmo suggests he name it "The Timmy" instead, and Billy takes his advice. When Timmy returned to the future, he found that Billy Gates ended up taking present Cosmo's advice and that the Internet was named after him (and Timmy was named "Internet").
In real life, Bill Gates is not really the inventor of the Internet, but in the Fairly OddParents universe, Billy Gates is referred to as such. Although Bill Gates is the founder of the company Microsoft and an important pioneer in the creation of personal computers in reality, he had little to do with the creation of the Internet.
In the episode The Secret Origin of Denzel Crocker!, when Timmy traveled back to the 1972, Cosmo and Wanda are Crocker's fairy godparents so that means Cosmo and Wanda were assigned to Billy before Crocker because in "Father Time!", Timmy went back to 1970.
Dee Bradley Baker voices Billy Gates, who is listed in the credits to Father Time! as "70's Kid".