Can you talk more about non-fictive headmates, specifically how to distinguish them from fictives? Because I have a headmate who has refused to identify herself for the longest time. Also, is it possible to turn an imaginary friend from one’s childhood into a headmate?

A non-fictive headmate is pretty simple anon ^^ 

If you have a headmate and you don’t immediately know what canon/source they are from, then they are most likely a non-fictive headmate.

Non-fictive headmates have their own personalities, histories, and appearances, just like fictive headmates. The only difference is that they don’t share their identity with someone from fiction.

Non-fictive headmates may, like fictives, have had lives before sharing a mind with you. They may be lawyers, teachers, space-pirates, school children, or woolly mamoths (or frankly anything). But it is also possible for them to only have experienced the world for the time they were with you in which case their history will be the same as yours, from the time they came into existence.

The best way to get to know a headmate is to talk to them. Ask them about themselves. Their names If they come from somewhere else, what they remember, what sort of tea they like :p

In most cases you will immediately know if a headmate is fictive because you will be familiar with them. In some rare cases, you may have a headmate who you think is non-fictive, and later you read/watch something and discover that they actually are a character from that. But that is very rare. Mostly people only encounter fictives of people they are familiar with.