A soulbond is an externally originating headmate with a fictional origin that has an emotional attachment of some kind to the soulbonder.
Soulbonds can be: canon fictional characters, AU versions of fictional characters, RP or story characters that you have written, or written by someone you know.
Soulbonds are not tulpas, they are not created in the mind. They have an existence outside of the soulbonder’s head, and the soulbonder is merely host to them in some way, like a channeller with a spirit.
Soulbonds always have some emotional attachment with the soulbonder, generally having the soulbonder’s best interests at heart. The soulbond may feel romantic toward the soulbonder, or be a guide or mentor. Even villainous soulbonds seek to help and care for the soulbonder, though they may do so in villainous or actually unhelpful ways.
Soulbonds are (generally) sure of their identity as the fictional character they claim to be. They have memories of the events that happened to them, they think and act as themselves. They may even be able to go back and forth between the soulbonder and their homeworld.
Soulbonds tend to link up to people in one of three ways.
1) Permanent resident: The soulbond attaches to you , and moves in completely, becoming a new, permanent headmate. They are unable or unwilling to return to their original realm, and are now a part of this one.
2) Day tripper: The soulbond links up to you, and uses your mind like a time share. They go back and forth between their realm, and this one. Sometimes they can communicate from a distance while they’re in the other realm, sometimes they can only talk to you when they’re here.
3) Internal Cellphones: The link between you and your soulbond is like a permanent phone connection. You are here on earth, doing your thing, and they are in their realm living their own life, and the two of you talk back and forth as if over a shared phone mental phone line.