So I was looking into theories about Fading away in the Warrior cats series like; They're in a 2nd Starclan, They don't exist anymore etc. But I wasn't satisfied with any of them, so I was like "I'll just make one of my own!". And here we are! It's a bit complex but hopefully, you'll be able to understand it.
If you don't understand anything, just ask. What faded cats are:
A faded-cat is a spirit in their purest form. They can not physically affect the world around them in any way, neither can things like gravity affect them either. All cats, no matter who they are will eventually become faded. A cat becomes faded once they are completely forgotten or being given fetal wound as a non-faded spirit, which simply speeds up the process. Being a faded cat is like being in your own "personal heaven or realm of existence", no one to bother you.
Faded cats still exist but just completely invisible (No scent, make no sound etc.). They're best described as being a 'presence'/'wandering consciousness' but nothing more. In this state, they forget everything; they're memories, who they're family were and even their own name. They show little emotion or personality, being completely at peace with themselves and the world around. They have no concept of self or time.
Faded cats are extremely knowledgeable and wise. They have the ability to go into Living cats'/non-faded spirits' heads (Possessed) and see their; Memories, dreams, feelings, thoughts, and perspective (Like Jayfeather's power). Through the post won't be aware of being possessed. Faded cats also can't affect the possessed cat, but only observe them. Faded cats aimlessly wander the Earth/Realms of the afterlives, until they find something that peaks their interest. Though cats who fade-away in the Dark Forest will usually remind trapped there for some time, struggling to leave it due to the Dark Forest's endless and tricking nature. However, they don't seem to mind this.
It's impossible for a living cat to ever see a faded cat, the best thing they can do, is "feel a slight presence of a faded cat". However non-faded spirits are able to sometimes catch "glimpses" of faded cats but this is very hard and rare. Only those who are very knowledgeable and skilled on spiritual powers are able to do this. No one has ever managed to talk to a Faded cat. Faded cats also can't talk to one another or even aware of each other. Reincarnate:
After many generations, all faded cats will eventually reincarnate. They do this by possessing the lifeless body of unborn kit. If they do not have spirit/soul inside of them at birth, the kit's body will never be conscious and will come out as a still-born. However this doesn't tend to happen due to the sheer number of faded spirits. Since the Faded-cat has already lost all sense of their former selves, it won't affect their new life in any way and they will develop a new identity. Sometimes the faded cat will even be the opposite gender from their old life. Unlike when Non-faded spirits reincarnate, who still have a strong connection with their former life due to retaining their old memories.
However rarely a kit may be processed by two separate spirits (More than 2 is extremely rare). Depending if the spirits are faded or not, different things can happen. If two non-faded spirits process the same kit, it can develop into a split-personality disorder due to the clashing, separate, already established personalities. If they are both faded cats, it wouldn't make a difference as the two personalities will develop together and coexist.
Though in the cause of Cinderheart, she was being processed by Cinderpelt, a non-faded spirit, and a faded cat. Since Cinderpelt already had an established personality, the faded cat simply developed its new personality around what was already there. To the point, it seemed that both spirits' personalities were almost identical, only that the Faded cat side lacked Cinderpelt's memories. So when Cinderpelt felt Cinderheart's body, there was still Cinderheart's unique spirit inside of her for her to live on. (In usual circumstances, if a spirit left their body, the body would drop dead due to there being no spirit/soul inside of them.)