Brittany Tenelli is a major character in the franchise. She is one of Cindy's best friends and Carl's female counterpart. She is often seen singing, dancing and hanging out with Cindy and Libby as a third member of their circle of friends in many episodes, but usually appears as a supporting character.
Brittany is around the same height as Cindy and Libby and she is very pretty. She has blue eyes with red-pink eyelids and blonde hair that is worn in two long pigtail braids held together by four pink hairbands on both sides. She wears a white-and-pink midriff shirt, a pink short-sleeve jacket, pink pants with white and pink waistbands, and pink Mary Jane shoes with white socks. In the movie, she wears bracelets on both of her arms.
Brittany is very ditzy, clumsy, bubbly, bouncy, sweet, innocent, perky and kind. She is one of the few characters who doesn't antagonize Jimmy in any way and she appears to admire him and his brilliance. This makes her a comic foil to the brainy Cindy and Libby and one of Jimmy's few fans. She is very happy-go-lucky, and with her head in the clouds, she's often confused in a situation and is shown to be illiterate. Brittany is totally devoted to Nick, but of course, he can't devote himself to just one woman. She’s very passionate about the people in her life, and falls to pieces rather easily. Brittany often has low self confidence and sometimes gets picked on by Cindy and Libby when she tries to get involved with their escapades, but they still love Brittany and the three of them are very close.
- "They're just little puff balls!" -Attack of the Twonkies
- "What'll it be, boys?" -Jimmy Neutron: Boy Genius
- "Uh, I didn't do it!" -See Jimmy Run
- "Oh, my God! Those gems are so gorgeous!" -A Beautiful Mine
- "Excellent choice!" -Jimmy Neutron: Boy Genius
- Brittany may be a parody of Britney Spears due to her name, the fact that she likes to dance and sing, speaks with a valley girl accent and wears midriff outfits.
- It is revealed in Raise the Oozy Scab that Brittany is illiterate, which she sometimes denies.
- Aside from being one of Cindy and Libby's best friends, she seems to be close friends with Amber as well.
- Brittany is one of the only people Nick shows any form of love and affection to, along with his mother (not that he wants people to know, since it would ruin his macho reputation).
- Brittany's Planet Sheen counterpart is Princess Oom.
- She has a loud and high-pitched laugh that the characters find annoying.
- In the movie when the kids are reunited with their folks, she is seen speaking to an old woman and two adults, most likely her grandmother and her parents.
- In the planned season four, Brittany was going to have more episodes focusing on her.
- Sometimes, it is hinted that Brittany might have a crush on Jimmy.
- Brittany's appearance looks like Cindy's original design.
- Whenener she's excited, surprised, or frightened, her pigtails shoot up. This is seen in episodes like Love Potion 976/J and Return of the Nanobots.
- When she's sad, her pigtails droop down. This is seen in the movie.