Tightest implementaion of this concept I've seen - wizards-level writing (as someone who spends a lot of time with FFG's RPGs, that is a compliment). With the very professional presentation and art this wouldn't be out of place in an official expansion. I think it kind of 'gets' the way DnD does its classes - as archetypes that come to life in their implementation, rather than specific professions in the lore; this is something that other custom classes sometimes miss the mark on. I especially like the thought given to the Occultist variant. I imagine it'd be easier to say 'of course you can't multiclass into being a ghost', but turning the class into a kind of spirit beastmaster is a good way to resolve this without limiting creativity.
I'd recommend this to anyone interested in this concept, or who has a player who is more interested in supporting other players to do great things in combat rather than wrestling with the complexity of combat themselves. I took part in one of the playtest campaigns for this project (before I had seen the rules text for the class) and I witnessed our party grow a great fondness for our spirit guide, Thorn (Duty not done), a child who died before he could see the world. Hard not to make some strong character bonds when they're inside your head!
A couple of meagre quibbles/ suggestions after reading through:
The idea of character resurrection as a spirit guide to finish unfinished business is very interesting, but I feel like it deserves a bit more focus than a lil green side box.
The Ethereal Sight ability could use a bit more explanation - as someone less familiar with the DnD lore I wasn't entirely sure what 'seeing into the Ethereal plane' meant - I imagine new player would be in the same situation. An explanatory box would be really handy here.
[5 of 5 Stars!]