Because it dawned on me that I didn't get this to you, I thought I'd share the couple ideas I had for Ruby's supernatural savior!

I currently have two possible ideas for the entity. Maybe three, depending on what fits. Two story-based. The third is a backup for in case we can't make the first two fit into the rest of what you'd intended! And the story-based ones are, of course, open being changed if you have other ideas. 
 
1) This one depends on if Alex's resemblance to Brahm's wife was intended to be a play on actual reincarnation or just a strong blood-relation resemblance. If the latter, then... The realm of the dead is an odd place, where time has no meaning, so sometimes, visions of the potential future can sometimes collide with the present. Brahm's wife died violently, so perhaps her spirit wasn't at rest. She sensed that there was a way to end the suffering of her husband and even her (misguided *cough cough*) murderer, but if Ruby had died that day, then it wouldn't have come to fruition. Right now, it's not in my head as a "chosen one" type deal, mind, just happenstance, especially if she ends up getting involved with both hunters and the supernaturals, which could help lead Alex and the boys to some sort of good place... At least, a place where Brahm wouldn't be compelled to torture Alex to torture Vander... as much, at the very least. Like, maybe forcing a frienenemy truce or something, that ends up continuing beyond necessity because of events yet unestablished, as a thought.

2) Similar concept, but it does delve more into an almost "chosen one" vibe. The entity was some sort of Fate or some such (whatever would fit in overall), not the restless ghost of Brahm's wife, though in that case it would make more sense to come up with something that deepens that reason, since it’s an overarching Fate rather than a direct MC link wanting to help Brahm and Vander get over their hatred, and save Alex from eternal torment at Brahm's hand.

3) More of a back-up for if the above don't work, and it's rather simple: the entity really was just a helpful ghost that took pity on a hurt teenager--maybe someone who died because of a similar trap now long since gone. The main purpose of the ghost here would simply be to serve as a driving force for Ruby's quiet obsession, and help make her quicker to accept the whole "Supernatural things are real!" bit.

Let me know what you think, if any of that sounds even remotely good to you!