Timmy was created by Tabitha shortly after the show started. She used her powers to bring him to life from doll form, shortly before her powers were taken away. Even though soon lost her powers, Timmy never permanently reverted back to doll form -- he was able switch back and forth at will.
Timmy functioned as Tabitha's conscience on the show. His love for Tabitha was tempered only by his love for Charity, who he saw as pure and innocent. He hated what Tabitha and Hecuba were doing to Charity, but he knew that Hecuba and Tabitha could destroy him. His favorite hobby was making and drinking his martimmies.
Timmy became a real boy, his wish having been granted by the Angel Girl, and Tabitha began introducing him around as her great-grandnephew, Timmy Lenox. Soon he began a journey to find the Demon's Horn, so he could save Charity from Zombie Charity. With an unlikely ally (Julian Crane!), Timmy found the Demon's horn and brought it back to Harmony to save Charity.
Unfortunately, while Timmy was able to save Charity, he was not able to save himself. He lost his life in the battle with Zombie Charity, and his heart was donated to Charity so that she could live.