Tootie is similar to Sussie from The Amazing World of Gumball.
Tootie was introduced in the Oh Yeah! Cartoons episode "The Fairy Flu", in which she already had a crush on Timmy. Her appearance was redesigned in her Season 2 re-introduction "Boy Toy" to what it is today. Interestingly, in the comic The Big Surprise which was released in between these two episodes, Tootie's appearance is a cross between both her original and her redesign.
It was unknown if Tootie is her real name or nickname, but it is usually written everywhere in her room or on banners during her birthday, and in a deleted scene from Boy Toy, Timmy, Chester, and A.J. make fun of her name as well, so it is likely it's her real name.
Her name is possibly a homage to Dorothy "Tootie" Ramsey from the 1979-88 TV sitcom The Facts of Life. The Tootie in that show, played by Kim Fields, was also geeky, wore braces, and was the youngest of her group.
Tootie is frequently a victim of improper animation. In many scenes her glasses are missing their temple arms, or her black tights will be missing. From Season 6 onward her glasses are drawn more simplistically round and change between purple and black color, depending on the episode.
Tootie may be aware of Timmy's crush on Trixie, as shown in You Doo, when she was playing with the Timmy doll, unintentionally causing the real Timmy to act rude to Trixie.
Tootie has kissed Timmy six times: Once in The Fairy Flu, a number of times at the end of Love Struck!, near the end of Kung Timmy!, Timmy kissed her near the end of Birthday Wish!, and in Dread N' Breakfast Timmy kissed Tootie again but refused to do so on the lips. Timmy and Tootie finally shared a very long kiss on the lips in "The Fairly Odd Movie", which resulted in Timmy instantly being considered a grown up by Da Rules and thus losing his fairies for a short time.
The Fairly Odd Movie also showed Timmy being forcibly kissed by Tootie when they were younger, and established that this kiss is the only one that Timmy could remember. According character bios on movie's press website, Tootie and Timmy's childhood kiss was done on a dare, but this is never elaborated upon in the movie itself.
Tootie appears more in Season 7 than any other season.
Tootie shares some noticeable similarities with Sam Manson from Butch Hartman's second Nicktoon, Danny Phantom. Among them, Tootie and Sam both have black hair and purple eyes, their outfits look similar, they're both voiced by Grey DeLisle, and both girls are in love with (and ultimately end up with) their respective show's protagonist.
Tootie's personality is somewhat similar to Sierra's from Total Drama World Tour or Clamantha from Fish Hooks. She also sounds very similar to Berry from Fosters Home for Imaginary Friends, Lizzie Devine from Kids Next Door, both also voiced by Grey DeLisle, and Phoebe from Kid vs. Kat.
As of yet, it was never explained as to why and how Tootie developed her crush on Timmy in the series. In the Fairly Odd Movie, Tootie mentions that she has been in love with Timmy since 3rd grade.