Hi,
Imaginary friends, to the best of my knowledge, are a completely healthy appendage of a normal imagination. They come along for all sorts of reasons, but my son's seemed to come along when he needed a bit of an anchor in his life.
My now-nine-year-old had an imaginary friend for about a year between ages 3 1/2 and 4 1/2. His little friend was named Kid and he lived under the edge of my sons pinky fingernail. He rode on my son's shoulder, sat on his hat, hitched rides in his pockets... but he was always there, 'physically' with my son. He first appeared during a time of upheaval -- new baby sister, I blew out my knee and needed surgery, we moved to a new house across town and my son started a new pre-school. All in the same four months.
It was a lot for my little guy to take in, but Kid helped him out. My son would sit nestled in his bedroom chatting softly to Kid, who was perched on my son's pinky finger. I could always tell when something was on my son's mind, even from across the yard, because that pinky would come popping up. Kid never left my son, was always there to listen and never got upset. Kid -- and access to Kid -- represented the one part of life my son could control. He needed that.
One day, after we'd been in the new house a few weeks, I noticed something new -- I saw my son sitting on the garden steps, talking to the brick edging. Now that's odd, I thought, so I wandered over. He told me he was just checking on Kid. I reflexively looked toward his pinky, but my son corrected me. Kid had moved into the garden. He had a lovely little home in a niche formed where the brickwork was chipped and the flowers hung over. My son put his pinky down so Kid could climb on. He lifted Kid up to me and I welcomed Kid to his new, and very pretty, home. I told my son to go ahead and take as long as he liked getting Kid comfortable. He spent another few minutes, then came in.
I only 'saw' Kid a couple of times after that. By Christmas, Kid had moved on and so had my son. But it's amazing how far a little bit of imagination can take a child toward feeling secure in a changeable world. That little bit of faith -- just enough to fit under a child's fingernail -- is all it takes to heal the human soul. :-)