Face: Long face with high cheekbones. Large, intense eyes under heavy eyelids. Sharp nose. Full well-maintained eyebrows. Not particularly pretty to most, unsettling to look at to some.
Hair: Dark brown hair kept in a tidy low bun with strands framing her face. When freed it reaches halfway down her back, slightly wavy. If drawing, please note that it's a low bun and not a high one! ( Visual representation of hairstyle )
Eyes: Greyish blue.
Height: 150cm
Age: Adult
Bodytype: Slender limbs, overall petite build with a narrow waist and wider hips. Looks fragile.
Complexion: Extremely pale with prominent, permanent dark circles framing her eyes. Lips are a pale pink. Sickly looking.
Outfit(s):

• A simple, well-tailored black dress with long sleeves and a floor-length skirt. Buttons line the front. ( It's a mourning dress okay ) Decidedly antique and out of place. Lace-up victorian walking boots.
• A long-sleeved white blouse worn with a black ribbon in the front, a long brown skirt with a high waistline accompanied by a leather belt. Lace-up victorian walking boots. ( Here's a reference for this one too )
  Misc: Will never willingly reveal skin below her neck, often donning thin gloves to hide her scarred hands from view. But it's possible for the sleeve of her dress to ride up enough to reveal discolored skin marred with old scars and burns. Smells like lavender. Old fashioned mannerisms.

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Voice: Very rarely speaks but when she does it's with a gentle voice. Has a noticable eastern european accent.
Body language: Carries herself like she's sorry for being there and trying to take up as little space as possible to make up for it. Head is always lowered towards the ground, hands usually clasped demurely in front of her. Has trouble walking or moving properly if she feels anyone looking at her, movements stiff and awkward.
Social: Doesn't search out the company of others, instead roaming the halls and grounds of Hogwarts and the surrounding area seemingly lost in thought. Enjoys chatting with plants, magical creatures and enchanted objects seemingly unphased by the conversations being one-sided.
Visibility: Due to her nature as a ghost-like entity students might notice her appearing out of thin air, disappearing just as quickly. It's not uncommon to catch a glimpse of her so brief it leaves them wondering if they ever saw her in the first place, always accompanied by the light scent of lavender.
Engagement: Only makes contact with others if they're struggling, in pain or in need of comfort. Will completely forget her own insecurities if she witnesses injustice or mistreatment of people, animals or plants leading to the perpetrator being reprimanded by her.
Quirks: Extremely bad sense of humour.
Things your character wouldn't do: Won't drink alcohol under any circumstances. Usually completely unable to communicate with the opposite sex. Being rude or hurtful on purpose. Poor manners. Getting on a broomstick.
Animals: Has a clear affinity towards creatures and plants alike, easily interacting with them.
Animagus shape: A well-groomed small brown rat with a glossy fur coat. The form retains her blue eyes. Freed from the insecurities and worries associated with her human form means she's friendly and happy to interact with anyone who seems willing, happy to recieve treats and pets. Knows many tricks for some reason?
Misc information that might be useful: Ladies and gentlemen, this witch is dead! Or something akin to it. We just don't know. For the sake of making it easier for her to fit into narratives and interact with characters and the world around her I decided to make her something that's akin to a ghost but not...Quite a ghost? Unlike other ghosts roaming Hogwarts she isn't transparent and lacks the ability to float through walls. It's possible to physically touch her but she's very cold to the touch and leaves you with a lingering feeling of "I'm not supposed to be able to do that".
However! She's able to disappear and appear at will and if a person looks at her for too long they'll end up with a headache, as if their eyes and mind become strained at the sight.
Characters with memories of their Legacy versions will be able to recognize her and their instincts will alert them to the unnatural state of her being.
Descendant of Legacy characters might suspect she's connected to the witch of the same name who disappeared all those years ago.
That said, it's possible for a character to be completely oblivious to her ghostly nature so go ahead and let them assume she's just an odd witch if that suits your narrative!