Is God-Machine Chronicle stuff allowed?
- Yes: Get the book and check the Essence Thief numen on p229.
- No: though luck. Try to bribe your storyteller!
Handy link with an overview of Numen powers.
Where to find Ghosts:
- New World of Darkness Core rulebook p208: basic information
- Book of the Dead: way more info, but this is more directed towards the Underworld and how it interacts with other supernaturals.
- Geist: The Sin-Eaters: describes a new splat of mortals that made a deal with a certain type of ghosts (Perhaps you should check this; seems quite close to what you want)?
I'm sure there is more, but those are of the top of my head.
Check Regaining Essence on p135 of Book of Spirits. It's not "consuming", it's theft. If all Essence is drained away in this manner, the target falls into slumber (in which it stays until it gains Essence somehow).
In general ghosts only gain power by sticking around (see Numina section of the New World of Darkness Core rulebook p210). The God-Machine Chronicle book describes some way that Ghosts or Spirits become more powerful (see p216 & 217), but there's no detailed 12 step guide.
The only ingame way that's more or less described is the Old Soul Numen from Mummy: The Curse (see p209). But:
- This is a rare Numen (so check with your ST)
- Why would a ghost of that age be interested in you?
Have you checked with the ST whether he allows your Ally to gain power?