International university students who are usually from China or East Asia (but can also be from other parts of the world), who live in halls of residence with other students and stay in their rooms all day, don't socialise (apart from at Phantom gatherings) and often don't even introduce themselves to fellow flat mates.
On the odd occasion you may see a Phantom poke their head out of their door, however if they see any sign of human life they quickly retreat back in to the realms of their Phantom cave.
Usually you only get one Phantom every couple of flats, but for those unlucky people who are stuck in a flat full of them, our hearts go out to you.
"Hi John how are your flat mates then?"
"Not too bad, but I have a couple of Chinese phantoms, I never see them, they just stay in their rooms all day and never come out!"