Writing entities that share a body

Struggled to came up with a title for this as I'm not exactly sure how to describe it. But i'm writing a story wherein the protagonists summons a spirit to give him strength for his quest. Long story short the spirit accepts and becomes a sort of guardian angel that shares his senses and is essentially a second 'mind' inside his head.

What I'm strugglin with is how often the spirit should comment on things, how 'present' he would/should be. This spirit character is one of the most important side characters as he will be with the protagonist for most of his journey, but I also don't want to push his presence too much. I'm afraid of scenes not feeling natural anymore or not flowing as nicely if they are interrupted too often by the voice inside the protagonists head.

Any tips? Thanks!