An imaginary friend is an imaginary being that children make up for company and comfort.
The series features several imaginary friends, who are portrayed as real characters, but normally can only be seen by and only interact with the child who made them up. Starting with "Baby Kate and the Imaginary Mystery," imaginary friends appear in the “talking babies”-episodes and can interact with all babies and animals.
The normal rules of reality do not apply to imaginary friends. All of them can appear and disappear at will. Nadine can even do magic, though it only affects D.W.'s imagination. They also have the ability to reverse time.
When an older individual approaches, imaginary friends usually disappear. If the small child does not notice them, the older individual may see the child interact with empty air, e.g., when Arthur sees D.W. brush Nadine's hair in "Arthur and the Crunch Cereal Contest."
Imaginary friends are aware that they are imaginary and that their real-life friends will eventually get too old for them. They only know what their real-life friend knows; e.g., in "Baby Steps," Nadine points out that she does not know what D.W. did before age three. However, Nadine often tells D.W. things that D.W. knows, but does not want to believe at the time, thus acting as her conscience.
When imaginary friends are not busy with their real-life friend, they spend their time in Fanta-city, where they study the latest techniques in disappearing, reversing time, and how to be a good imaginary friend. 
List of imaginary friends
Crossing the line
- In "D.W. All Fired Up," Nadine tells D.W. that the Tibbles are passing by the house, which D.W. could not have seen from where she was sitting.
- In "The Lost Dinosaur," D.W. learns from Nadine that Platy is giving Nadine a headache. It is possible that Bud told D.W. that Platy was annoying and D.W. was playing along.