The most important characteristic of an IC is to continually apply pressure on the MC. They need to be present at every turn. If not them, someone or something that adopts their part of the argument.
There are a number of fun ways you can handle this. Firstly, if you’re into a genre-bender, this character can be an actual ghost or a ghost-like character that appears to the MC during times of stress. In Field of Dreams and Stars Wars: A New Hope the IC is a father-like figure–someone who triggers the same emotions that an absent father triggers. Another method is to ‘hand-off’ the role of IC to someone more appropriate in the story. In Shrek, the role of IC passes from Donkey to Fiona once she becomes available to play. In Old Man and the Sea, I’m pretty sure the IC was the fish, so…
Whatever the IC is, it needs to challenge the MC–i.e. inspire doubt. I prefer to call the IC the devil’s advocate, and I’ve found, once you remove the character element, that the IC doesn’t have to be a single entity, or even human. Although I do think it’s easier to be challenged by an anthropomorphic fish than, say, something without agency (like a rock or the absence of something).
Hope that helps.