It's quite common for children to have imaginary friends. I had one myself, and we had some great fun together. We usually put off these friends when we grow older, and so parents usually don't have to worry.
However, you might want to start to worry when the imaginary friend refuses to leave.
One of our characters have that problem, and here I thought I'd show you some of our inspirations for particularly stubborn invisible friends.
Schizophrenia is a theme that is very popular in movies and such, the best example is probably Tolkien's Gollum, who after being corrupted by the Ring develops an evil personality that tries to take control over him, like in this scene, portrayed by (a very animated) Andy Serkis:
Another reoccurring theme is people, particularly children, who see ghosts or spirits around them. Sometimes they don't understand that the people they see are ghosts, such as the boy in Guillermo del Toro's 'El Orfanato'/'The Orphanage'.
Whoever sees the ghosts can be used to them, but for others it can be a little creepy when someone talks to someone invisible.
"I see dead people."
- The Sixth Sense (duh)
I guess the next step here is when your imaginary friends actually become so vivid that other people can see (and feel) them too. An inspiration for us was the three Alessa-s in 'Silent Hill' (only watched the movie though).
In this movie a little girl is tortured so badly her hate turns into a demon that looks exactly like her (only dirtier and in need of sleep). The good part of her is reincarnated in another little, very innocent girl, who grows up with foster-parents, unaware of that her evil twin is currently wrecking her hometown and filling it with monsters.
Normal kid: Not so normal kid:
So, putting all of this together, you get Eliah. She's a tiny, clumsy thing who's really shy and socially awkward, but always sees the good in people. Unfortunately she was born with more power than her body can handle, and this creates a second personality that is pretty much pure evil. Yay. Eliah aslo sees dead people now and then, but the major problem is this second half that she has to keep under control at all times.
I can pretty much guarantee you more stuff on Eliah! Until then, here's a link to a short comic about her on DeviantArt: The Broken Mirror