Well put simply she is my OC Doom's soul mate Max's sister and Alexander's half sister.

But things aren't so simple are they?
Before we get into this let me put out trigger warnings

Rape
Manipulation
Abusive relationships
Violence
Suicide
Alcohol abuse
self harm
Self starvation (not anorexia but similar)
Pedophilia
Incest ( barely a mention and not with Luna)

*the start*
Please see the chapter on old god soul mates for this to make more sense

Luna was a albino bunny and Doom's soulmate or more accurately chew toy. After dying she would reincarnate usually occupying the body of a rabbit but also any other creature. She would not remember any of her past life, certain things could trigger these memories but it was fairly random.
Sometimes she'd go through life without ever remembering.
She didn't have too much of a personality at this point.

Then Doom's first body was destroyed in a fight with her brother. Doom her brother his soulmate and Luna all died in this fight, Doom killing Luna so they could reincarnate together and her brothers soulmate defending him.

(No the brother is not nightmare
Luna reincarnated as any animal from a wolf to a velvet worm)

*the village*
A very long time ago there was a little girl who fell in love with the big bad wolf. The wolf had been abandoned, their pack mates driven out by hunters unable to grab the last pup. The pup learned to pose as a dog a mother dog with two pups their age taking them in. Mother dog was tall brown and white with floppy ears and big eyes, the first pup looking similar except her markings were placed differently.  The second puppy however had markings a much redder color and ears that pointed straight up. The wolf pup was a chocolate brown color getting darker as it creeped down her legs and to her paws, with luminous green eyes. So began the life of mother dog Astra, Tallow and Green.