The background story of Kirby in the 2003 TV series corresponds precisely to the Mirage Comics template. Kirby lives in a leased office space in front of the cellar in April O'Neil's apartment. He is shown to be a very talented sketch artist with a large imagination.
Kirby first appears in the episode The King. His role is highly similar to that of his comics counterpart. In this world, Kirby found an odd-looking gem in a coal pile and attached it to his pencil. After doing this, everything he drew came to life temporarily, except a portal-like device. When Donatello found Kirby when helping his brother Raphael turn the water temperature hotter, they went through the portal, and found that everything he had drawn since putting the crystal on the pencil had become a world. The villains he had drawn were overrunning a city, and Donatello and Kirby managed to defeat all the villains. There was only enough time for one of them to get back through the portal, and Kirby forced Donatello back through the portal. Kirby had enough time to send a note to Donatello through the portal, however. April's background as a living drawing is rudimentary hinted at in the episode "Return of Savanti, Part 1", but never really formulated.