The thing we guard is a person - for a very loose definition of person - and She was, once, literally the First House, the collective soul of the living planet. She is the one who gave Lily the power over death, Lily said once (while drunk) "To fix things." Lily called her Gaia, which apparently was an older name for the planet, and also the theorized embodiment of its soul. Gaia created - or perhaps nurtured - Humanity in the first place. If anyone can be called God, it is her, though she rejected any such label in the time I knew her.
Gaia died. Lily blamed Humanity; if she was very drunk, she blamed herself. Lily tried to Resurrect her. According to Lily, she fucked up big time, because a) she had never Resurrected anyone before and had no clue what the fuck she was doing; b) Resurrecting a planet is really fucking hard and she still hasn't figured it out; and, inferred, c) she was having a panic attack and still reeling from the death of everyone except some assholes who stole the lifeboats and left everyone else to burn, which she may or may not have been the proximate cause of. I have yet to get her exactly the right amount of drunk for a straight answer to that question.
So, if Lily can be at all compared to a Lyctor, she is the cavalier in this equation, and Gaia is the necromancer.
The woman I knew was not Gaia, not really. Lily called her Alecto, after a mythical daughter of Gaia, a Fury of unceasing anger who punished humans for their moral crimes. I... Liked her. Irene and Valancy, and then Irene Valancy, liked her, though I'm pretty sure all two and a half of them liked her in a "I can make her worse" kind of way.
The other Lyctors feared her, and at times hated her.
She was angry. And every time she lashed out, had a tantrum, went on one of her rampages, painted the world with the blood of the unrighteous, however you want to word it, Lily sank deeper into depression. This wasn't like her, she claimed, and... It was obvious over the centuries that Alecto was changing, getting worse - even that psychopath Irene Valancy was disturbed by the end, as Alecto's personality faded into her own despair and rage. Lily blamed herself for fucking up the Resurrection.
Mercymorn argued that Alecto was a monster in need of containment (though Mercymorn actually said 'put down like a rapid dog'); Cassiopeia argued she was miserable. The others mostly agreed with Mercymorn.
In the end, Alecto agreed to - or at least didn't resist - her confinement in the Tomb. Lily's plan was for her to sleep until Lily could figure out how to fix things; I'm not holding my breath. Lily entrusted me with this knowledge as her closest confidant and chose me in part because of that - that I alone understood Alecto.
Also, I'm the only one who fucked up Lyctorhood nearly as badly as Lily, so the Resurrection Beasts weren't trying all that hard to kill me.
I tried for Perfect Lyctorhood - tested out my own version of Lily's work on perfect Resurrection - and really, really fucked up. Samael died, and I lived a half life, prolonged but shadowed. This study contains all of my notes, and all I know of what Gaia and then Lily did. It wasn't enough for me. With any hope - because I doubt Lily has successfully fixed this in her own time - it will be enough for you.