Brittany Tenelli is one of Cindy's best friends. She is often seen singing and dancing with Cindy and Libby as a third member of their circle of friends and hanging out with them in many episodes, but usually appears as a supporting character. Brittany is very ditzy, clumsy, 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.
Brittany is around the same height as Cindy and Libby and she is very pretty. She has blue eyes with red-pink eye shadow 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 top, a pink short-sleeve jacket, pink pants with white and pink waistbands, and pink and white sneakers. In the movie, she wears bracelets on both of her arms.
- "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
- Being a blonde girl named Brittany who likes to dance, sing, talk like a valley girl and wear navel bearing outfits, her entire character may be a nod to Britney Spears.
- 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).
- Her 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 and like Nick, she would've become a main character.
- Sometimes, it is hinted that Brittany might have a crush on Jimmy.
- Brittany's appearance looks like Cindy's original design.