Brittany Tenelli is one of Cindy's best friends and Nick's girlfriend. 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. She bribed Libby into being her science partner with a shiny new music CD in Raise the Oozy Scab because she doesn't know how to read or write. She is very ditzy, clumsy, sweet, innocent 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. 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. 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 Jimmy Neutron: Boy Genius, 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 and wear navel bearing outfits, her entire character may be a nod to Britney Spears.
- She is illiterate, as revealed in Raise the Oozy Scab.
- 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 or emotions to, along with his mother.
- Her Planet Sheen counterpart is Princess Oom.
- She has a loud and high-pitched laugh that most of 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.