Demons are real they can also take a human appearance.
These are the so called shape shifters.

They are the jins as described in Islam some are good most are bad just like human beings.

They live in a parallel dimension they don't come from a different planet.

Demons can not hurt you when you belive in GOD THE ALMIGHTY THE MOST MERCIFULL but Jesus (peace be upon him) is a messenger of GOD THE ALMIGHTY THE MOST MERCIFULL when one of these demons approach say ALLAH SUBHANE WE TAALA.
Even if it was the devil himself he will instantly disappear when hearing the name of GOD THE ALMIGHTY THE MOST MERCIFULL.