In Harry Potter, they call that person a "Secret Keeper", although that also involves magic and whatnot.

In computer terms, a group of people who are entrusted with secret information (such as SSH keys) are indicated to be a part of (or within) a circle of trust.

In High School, girls call those people BFFs.

More colloquial term for "confidant"
A very effective set piece can be conflict with the spirit world.  A nice example of the kind of thing I am thinking of is from the 1982 Conan film, where the wizard opens a gateway to the spirit world to get demons to revive Conan, and where Valeria fights incorporeal demons to save Conan (YouTube, from 6:45 into the clip to the end &amp; onto next clip).

RQ3 had nice mechanics for dealing with these situations, with shamans who can take characters to the other side through their fetch, spirit combat to handle attacks by hostile spirits, using magic points as an orthogonal measure of strength to hit points.  

AD&amp;D was pretty lousy for this kind of thing back in the day: its distinction between combat involving the incorporeal attribute or in the ethereal and astral realms didn't really work for setting up situations.  But newer D&amp;Ds seem to have better resources, though I've not seen this in action.

What good game mechanics are there for handling conflicts with spirits?

Postscript

The kind of thing game mechanics need, I think, to sustain interest in spirits and spirit combat are:


Spirits are immaterial and that means you have to do different sorts of things to influence them.  E.g., if the best way to deal with an ancient ghost is to chop it up with an axe, that'll spil that atmosphere a bit;
Likewise, spirits are potentially powerful adversaries.  Not very high-powered spells that can get rid of most spirits with a 65% chance of success detract from interest;
Characters can be experts in spirits, and it is good if the nature of those experts draws on resonant real-world and fictional atmospheric devices such as shamans, ancestor worship, fetches, and the like.  It's also good if characters regularly need the services of these figures. 

