I took the spitball to the back of the head, and now I feel a lot smarter.
— Billy in Doppel-Gamers
Billy, like the rest of the Thundermans, has superpowers and is aspiring to become a superhero. His superpower is Super-Speed.
Billy was born mid-air on the way to Metroburg Hospital as revealed in the episode, A Hero Is Born. This could explain why he is not as smart as the rest of the family. He moved to Hiddenville with the rest of the Thundermans and enrolled in Hiddenville Elementary School with his sister Nora.
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 thoughts.
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 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. He came up with the name, when he saw a jar of Luigi's Marinara Sauce on the counter. It doesn't take Nora or Chloe very long to find out that "Luigi" is just Billy dressing up like someone else.
Billy has had multiple life-threatening moments. He almost dies Thundersense; after bouncing on a trampoline with his Super-Speed and getting stuck on a breaking gutter on the roof. Luckily, Phoebe saves him. In Shred It Go, Billy almost got shredded by the shredder machine until Nora came in to save him. In Thunder in Paradise, Billy almost gets sent to the sea as a result of Phoebe needing a distraction to go to a volcano.
The Thunder Twins turn Billy into a rat in The Amazing Rat Race so that he can win a rat race for them. However, he enjoys being a rat more than being a human and therefore refuses to turn back to human. He only accepts to turn back to human after realizing that Nora is afraid of rats. During the flamingo-flu season in Phoebe vs. Max: The Sequel, Billy starts to turn into a flamingo because he's afraid of taking the superbug. But with Chloe's help, he gains the courage to take the bug.
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. After the superpowers are restored, Billy gets Nora's Heat Vision but doesn't want them because he thinks he's not cut to be a superhero. However, when the supervillain Fairy Pinch-ess captures Nora, Billy gets the courage and saves Nora. He then helps get the rest of the people to safety.
In Smells Like Team Spirit, it is revealed that Billy is easily gullible. Dr. Colosso uses this to take advantage of Billy and make him steal fresh vegetables for him. Nora later helps him figure out that Dr. Colosso is using him and they work together with Chloe to get back on Colosso.
Billy starts superhero training with Nora in 21 Dump Street after re-enacting how they defeated Fairy Pinch-ess to prove that they're ready for training.
By Z's All That, Billy has learned how to use his superspeed to spin his hands really fast causing the wind around to form some sort of a tornado. This trick is called "armnadoes". Phoebe and Max are so impressed by Billy's armnadoes skill that they decide to drop Nora and choose Billy as their Lil Z (for the Z-Force Lil Z Mentor program). They help take Billy's armnadoes to the next level by showing him how to make them more powerful and bigger. They also help him learn how to spin his body around, forming a vertical tornado. When Nora learns what happens and comes to face off her siblings, it leads to a huge disaster which leaves the Thunder Twins trapped under a giant pizza and a guard tiger ready to attack them. Nora saves them from the pizza and Billy saves them from the tiger using his armnadoes. The twins agree to train both Billy and Nora as their Lil Z's. They apologize to Billy for pushing him too hard.
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.
- Main article: Billy and Nora
- Nora is Billy's younger sister, and even though Nora constantly shoots lasers at Billy and sometimes is annoyed, they have a good relationship. Billy thinks of Nora as his best friend. When Nora was accepted in a different school, Billy was devastated until he found out that she was not going to attend anymore.
- Chloe is Billy's youngest sister. They hang out a lot with Nora. She is probably more intelligent than Billy as seen in many episodes. The two of them, and Hank, are the fastest travelers in the world with Chloe's teleportation, Billy's super-speed, and Hank's flying.
- 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.
- Phoebe is Billy's older sister. They are similar in that they are both naturally friendly and are terrible at keeping secrets. They don't hang out very much, and Phoebe has sometimes manipulated Billy for her personal gains. Billy often does tease Phoebe as well.
- In Cookie Mistake, Billy and Phoebe have the time to bond in Sweet Gam Gam's Cookie Palace.
- Hank is Billy's father. While Billy does think that he has somewhat of a lame personality, he really looks up to him. They are also the only ones who know that he mailed Nora to Abu Dhabi right after she was born.
- In You Stole My Thunder, Man, Hank wanted to spend time with Billy so they collect rocks because Billy realizes that he likes gathering rocks.
- Barb is Billy's mother. Billy thinks that she is somewhat lame, and he looks up to her a lot less than they do Hank.
- Superhuman Speed: Billy has shown the ability to move at unreal speeds with ease. Although he is capable of generating a force along with him that he can use for various effects, he has not been able to control it and really doesn’t know about it.
- Cyclone Spinning: Billy is capable of generating cyclones by just super-speeding in circles at extremely fast speeds. This can be used as a vacuum or a fan-like effect, allowing him to suck up small objects and repel other objects with ease.
- Heat Vision: Billy accidentally gets Nora's main powers instead of his own superpower when Max returned his power with a short-circuit orb. However, Billy gets his powers back by Thundermans: Banished!.
- He can't keep a secret only because he isn't so bright.
- He thinks castles are lame.
- He has imaginary friends who he talks to when he's not with Nora.
- Originally, when they were creating the show, his name was going to be Oliver.
- When Barb and Hank were going to the Metroburg hospital, Billy came out early which means he was born in midair.
- He doesn't usually wear underwear as mentioned in multiple episodes including He Got Game Night, One Hit Thunder.
- Billy's voice has gotten deeper since Season 4, because of his actor Diego Velazquez going through puberty.
- His birthday is June 22nd, which means his zodiac sign would be a Cancer.