The changeling faerie creature who copied Tim Hunter was created by John Ney Rieber and Peter Gross in The Books of Magic issue 43 in 1997.
Major Story Arcs
The changeling came from Faerie. It was given power and memories borrowed from Tim Hunter by the Wobbly, a creation of the real Tim's. The Wobbly was able to get the power because Tim gave it up.
The changeling takes over Tim's life at home at a time when Tim is away in America. His family doesn't realize Tim has been replaced.
However, one of Tim's other creations, Awn the Blink, does recognize the changeling as not being Tim. The Wobbly, in the form of Tim's old owl Yo-yo, alters Awn so he believes the changeling is Tim.
The depressing thing is that the changeling has a better relationship with Tim's dad and his fiance Holly than Tim does.
The changeling goes to the cemetery where Tim's mom Mary is buried. There is a strawberry plant buried there that contains Mary's memories (because the strawberries came from seeds given to Tim by Death). He uses them to further feel like Tim.
Soon after, at the wedding between Bill and Holly, the bulk of Tim's magic returns, reverting the changeling to his natural form. The Wobbly shows up as well, and tries to convince the changeling to throw away his dad with Tim's magic, and then the Wobbly will make him love the changeling, which is all that it wants.
However, the real Tim shows up and convinces the changeling that the Wobbly is acting in its own interests. The changeling uses the magic to destroy the Wobbly. Now he just wants to go back to Faerie.
Tim, however, is unhappy that the changeling has his magic, and gives him some peanuts to eat. They are from another realm and destroy the changeling and release its hold on the magic.