Jinn

I am not aware of any classical sources that specifically identify Za'far the jinn as coming to Karbala, although this story became popularized later. However, there are narrations which indicate that some of the jinn sympathized with or sought to aid Imam Husayn (A), and so the story of Za'far the jinn can be seen as reflective of that general idea. 

The Qur'an and hadith (both Sunni and Shi'i) give the sense that the jinn exist in the same world as us. The Qur'an and hadith describe occasional interactions between humans and jinn on a physical level - for instance, seeing a jinn manifest as an animal (as in the story of the jinn who came to see Imam Ali in the Mosque of Kufa) or hearing their voices audibly. Another example would be the jinn working for the prophet Sulayman (A).

However, apart from those occasional interactions, usually jinn keep to themselves and human beings do not see them.

However, some mystics have held that there is a sort of veil between the human world and the jinn world, which you could call the barzakh. That is, humans and jinn subsist in the same realm, but in something akin to parallel dimensions. I am not aware of any basis for this in Qur'an and hadith, but it could be true. 

So do jinn live in the barzakh? To me, the best view is "maybe" and "it depends on what you call the barzakh". If one takes "barzakh" to mean something akin to "mundus imaginalis" - that is, the imaginal realm which humans access through dreams, visions, or imagination, it might be correct to say that jinn live in barzakh. After all, people usually encounter jinn in dreams or the inner realm (for instance, Shaytan whispering to the heart), rather than in physical life. This is similar to how people are more likely to have a dream or vision of a deceased person, such as Imam Husayn (A), rather than physically seeing them walking down the street.

If one takes barzakh to mean specifically the place where human spirits go after death but before the Resurrection... while this meaning of "barzakh" may have some relationship to the above, I am not aware of any texts which specifically say that deceased human spirits live in the same place as living/deceased jinn, although I don't think we have any texts that rule out any sort of interaction ever between living/deceased jinn and deceased humans.

In any case, it seems as if the existence of the human being after death is more similar to the jinn, because the human being in the barzakh is less encumbered by the physical body and able to move around more.

In fact, I don't think we have any texts that say what happens to jinn after they die but before the Resurrection - for instance, do they go to their own barzakh? - however, the Qur'an says that, after Resurrection and Judgment, the jinn who go to Hell and the humans who go to Hell are all mixed together in the same Hell. It does not specify whether the jinn who go to Heaven and the humans who go to Heaven go to the same Heaven.

Anyway, it can be quite difficult to make concrete statements about spiritual cosmology, metaphysical realities, or other things which are not tangible. Even if someone says something, it is really quite difficult to prove whether it is true or false, except in the case of Qur'an and accurate hadith, which we accept on the basis of them being divinely inspired or approved.

So, this, in short, is my understanding of what the Qur'an and hadith say, and also what some mystically inclined thinkers have said. God knows best!

The idea that every human being is created with its own associated jinn, called a qareen or hamzaad, is mentioned in Sunni hadith. This jinn is usually considered to be an inherently satanic spirit, and its job is to misguide people (except for the Prophet (S), who is said to have tamed and converted his). In the Sunni tradition, this idea is typically taken as an interpretation of Qur'an 43:36.

To my knowledge, there are no hadith through Shi'i chains which speak of the qareen. (These hadith appear in Bihar al-Anwar, but they are the Sunni hadith, not separate Shi'i hadith.) Ayatollah Nasser Makarem Shirazi dismisses the idea of the qareen as superstition. 

This is not to say that no single Shi'i person accepts the idea of the qareen, as some Shi'is may embrace these hadith. However, it isn't considered a fundamental belief or requirement in Shi'ism to believe that people have a qareen or to interpret that Qur'anic verse as speaking of a qareen. 

As with most matters of the unseen world, God knows best. 

The Qur'an itself does not discuss the topic of possession, in the sense of a non-human being taking over a human being, apart from emphasizing that the prophet is not insane/possessed (majnun). That it, it seems, at the time, people would receive or claim to receive messages from jinn or other beings, perhaps sometimes while possessed. The Qur'an does not argue that this is impossible; rather, it argues that the Prophet is above this, and that the revelation is directly from God.

Beyond that, one can glean from the fact that the Qur'an mentions humans and jinn as two halves of creation (thaqalayn) that there is some reason for us to know about each other and some sort of interaction, even though it is usually not visible. 

In practice, possession - in the sense of ceding one's self to an external being - is extremely rare, except in religious traditions where people put forth considerable efforts to induce this condition. (Of course, one can debate whether this is actually happening or not.) 

For most people, the most that might happen is that they might be somewhat influenced or affected by a being external to them (such as the waswas of Shaytan). If this does happen, it is not necessarily a sign of moral weakness or sinning; there are all sorts of factors that could contribute to it. (However, violence and licentiousness may also be contributing factors) Similarly, salat and du'a helps to avoid or mitigate this, although it is not an absolute guarantee that it will not happen. 

In any case, it is better not to be paranoid about these things unless one actually has a solid reason to think they have a problem. How often do jinn get blamed for the acts of men!