It's said that Soulbonding is unique with every person, and I can attest to
that. I have various Soulbonding idiosyncracies, like the fact that I
Soulbond men almost exclusively, while I'm actually distrustful of the
opposite sex in real life. I also don't have so much of a mind-room, but
involve myself mostly with the Soulbonds in their native habitat, as in
active story-brainstorming. And yet, they are definitely still Soulbonds.
I have my own ideas about what Soulbonds are. Like most Soulbonders, I believe that my Soulbonds are truly alive and not just made-up things. I've heard a lot of people propose that story-worlds really exist, and story-makers "channel" them from wherever they really are. This is a great explanation for some people, but it doesn't quite work for me.
My own belief is that a thing that has love, devotion, or even plain fascination poured into it gains a sort of spiritual life. In our era of mass communications, where fictional characters can reach millions of people, massive amounts of life are poured into these characters, and I believe that this gives them life as spirit-entities. I see Soulbonders as people whose love and fascination for a character is so great that it gives spiritual life to their own vision of a character (reality, especially in fiction, is subjective. No two people have exactly the same take on a given character), creating a Soulbond, who is at once an aspect of that character's spirit, an aspect of the Soulbonder's personality, and a unique person with a will of their own.
Thus the act of creation (often including Soulbond interaction) is reciprocal. Characters and Soulbonds do have their own will, and will often tell an author (or fan-author) large stretches of their story that can largely be taken at face value, and there's a sort of sacred trust between character and author that is destructively violated if the author pushes the character to do things against sa's nature and will. But at the same time, the writing is still very much a creative endeavor, and the author must make creative choices, which, if made with responsibility and respect, develop and strengthen the character's personality and continue the cycle.
I know that not everyone feels they can believe in "spirits" in a literal way, but hopefully this will at least be a helpful model for thinking about Soulbonding for some.