The introduction of a new character in Season 10 was first teased in May 7, 2015 when Butch Hartman posted a picture on his Facebook with a never before seen promotional poster behind him, and a piece of a new character's blond hair could be seen. In August 17, the show's creator announced Season 10 has been in production, and confirmed it would feature a new character, that would be Timmy's new neighbor. The silhouette of the character was revealed. 🌌
The next day, the new season was formally announced via press release, and the new character was revealed to be called Chloe Carmichael, a girl with whom Timmy will have to share Cosmo and Wanda. Her look was revealed alongside the announcement.
Chloe replaced Sparky due to the character's negative reception.
Based off of some of the concept drawings, it appears Chloe's bow (and sandals) were originally going to be pink, before they were eventually changed to purple.
Due to the definition of overachieving, it shows that she is excellent in academics, among many other areas. This can be seen in the many trophies and first place ribbons in her room. 
She knows 12 languages, including communicating with dolphins, according to this sneak peak clip released by Butch Hartman of the 10th season
The languages she is currently known to speak are: English, Russian, Dolphin, and Flightless Booby Bird.
It's implied she could be good at cooking, as she made lemon squares for everyone in her debut episode, as well as Tofookie in The Fair Bears.
She on multiple occasions is shown to be a poor actor and performer. Her attempts to perform her one woman show, "Totally Chloe", are mostly met with annoyance from other characters.
She is a vegetarian. However in "A Sash and a Rash", after wishing herself in a slacker, she is shown eating bacon. Based off of her parent's reaction to seeing her (Timmy in her body) eat meat in this episode, that could indicate she's only is a vegetarian to please her parents, hence why she was okay with eating it when she's not stressing out.
Chloe having a gap in her teeth mirrors Timmy's buck teeth.
Like Timmy, Chloe regularly sees a therapist, who she refers to as her "talking doctor". 
Chloe briefly takes on a Dinkleberg role for Timmy. Both Chloe and Dinkleberg do nice things for people, and both are hated by a member of the Turner Family out of jealousy. (However, Timmy only hated her in the beginning of the episode, "The Big Fairy Share Scare".)
She is the only main character besides Timmy to not be absent in any episodes after her debut. Cosmo and Wanda were both absent in "The Big Scoop!" in Season 3, Poof began being absent in many Season 9 episodes and the entire tenth season (except "Certifiable Super Sitter"), and Sparky was absent in three Season 9 episodes and was taken out of the show by Season 10.
Her full name is either "Chloe Mahatma Gandhi Carol Burnett Carmichael" (as stated in Tardy Sauce) or "Carol Mother Theresa Neil Armstrong Carmichael" (as stated in Booby Trapped and Which is Wish).
Rugrats - Chloe has the same last name as Susie Carmichael from fellow Nicktoon, Rugrats, however, it is not intentional, as stated below. This is later referenced on-screen in the episode Animal Crockers, when Chloe briefly wears a dress closely resembling that of Susie's main outfit. Also her characteristics somewhat resemble Susie Carmichael.
The Lucy Show - Butch Hartman would later mention that he got his inspiration for Chloe's last name from The Lucy Show's titular character, Lucy Carmichael.
Having nervous breakdowns when overwhelmed by emotion
Making disastrous wishes
Grabbing onto and/or shaking other people when excited or nervous
She is obsessed with helping other people.
She's goes to school even on weekends and holidays, even goes to school just to touch it on contact