Billy is not the smartest kid but Nora helps him figure things out, both in school and at home. In Mall Time Crooks, it's revealed that the Thundermans take turns in explaining things to Billy. After Billy's fight with Nora in Are You Afraid of the Park?, Billy uses Max to feed him thought.

He is the middle child and the youngest son of Barb and Hank Thunderman. Billy, like the rest of the Thundermans, has superpowers and is aspiring to become a superhero with superhero name Kid Quick. His superpower is Super Speed.

You're his younger sister and he struggles to be a good brother. In Adventures in Supersitting, it is revealed that Billy doesn't have other friends besides Nora and this has been repeated multiple times throughout the series. Billy thinks of Nora as his best friend. Billy worries about losing Nora in Paging Dr. Thunderman when he thought Nora was transferring to another school because he would have no one to play with. In Exit Stage Theft, Billy makes up a fake friend called "Luigi Marinara" when he gets jealous of Nora spending more time with Chloe than him. It doesn't take Nora or Chloe very long to find out that "Luigi" is just Billy dressed up like someone else.

In Thundermans: Secret Revealed, Billy fails the Hero League aptitude test, getting only 22% because he answered "triangle" for everything. Nora feels bad for him and swaps her high score with Billy's, making him think he is very smart. But when Nora realizes that he is at risk of hurting himself, she tells him the truth. This, in addition to Max taking away his powers, makes Billy think that he will never be a superhero.

Billy stands out from the rest of the Thundermans family in that he is eager, sweet, friendly Billy is very dim-witted, but he is always cheerful and optimistic. While his siblings are manipulative and competitive, Billy tries to help them and agree to their plans. Phoebe, Max, and Nora often take advantage of his sweetness to make him do things for them. Billy has a hard time keeping secrets and ends up exposing them accidentally, most of the time.

Billy is said to be able to speed around the world in 1 minute, but he's not faster than lightning, in order to circumnavigate the world in 1 minute, moving along the equator, would need a speed of 1,494,060 MPH, whereas circumnavigating the world from either the North or South, would need 475,080 MPH (Due to Earth not being a perfect circle), lightning travels an average of 270,000 MPH.

Max is Billy's older brother. He and Max are kind of similar. They hang out on and off, but they are not the nicest to each other, and Max often manipulates Billy for his personal gains as he's easily gullible. However, Billy does look up to him a little bit. When he and Nora got into their biggest fight ever in Are You Afraid of the Park?, he turned to Max. When Billy was planning on retiring in Not So Fast and Furious, Max talked some sense into him which made Billy decide not to retire and helped Max and Phoebe take down Kitty Klaw.