One of my favorite tropes, which probably harkens back to how much of an impact I Will Fear No Evil had on me, is the voice in the back of the head. In at least three of the books on my list, including Mail Order Bride, this takes the form of an AI that also happens to inhabit the main character/one of the main character’s bodies.
There’s a lot of reasons this particular trope appeals to me. For one thing, it gives the protagonist a constant companion. Someone who both cares about their wellbeing and can be completely honest with them. For another, the possibilities for banter are wonderful. When you have a character that can keep up a running commentary in someone’s head without anyone else hearing, the things the character can get away with saying are absolutely hilarious, so the character is great for comic relief. But perhaps the most important thing is, the voice in the head can be honest with the main character in a way no other character could.
There is something incredibly fun about writing two characters who literally can not walk away from each other, but it does also have its difficulties. You can’t ever completely isolate the main character, because they always have the friend in their hear, whispering encouragement, giving them a reality check, letting them know that whatever happens, they aren’t alone.
That last bit is important. If you include this kind of character, then the protagonist is never alone, and God, that’s a powerful idea, and one I really enjoy exploring.