Physical Description:
Olivia O’Brian has a heart-shaped face, smooth dark skin, and deep brown almond-shaped eyes. Her nose is on the larger side, and her full lips frame a sharp chin. Her black curls are thick and wild, falling well past her shoulders. Most of the time she wears her hair loose or tied back in a messy ponytail — not because of style, but because she doesn’t have the patience to do much with it.
At around 140cm, Olivia is average in height for an eleven-year-old, though a bit curvier than most. She feels perfectly comfortable in her body, and that confidence shows.
She hasn’t developed much of a fashion sense yet. Although her mother is a witch, Olivia usually wears Muggle clothes — often in shades of purple and always chosen for comfort. She rarely wears jewellery at this age, and her robes are usually crumpled. Her tie is often tied poorly, if at all.
Olivia carries herself with noticeable poise and confidence. This can sometimes come across as arrogance, especially because she often holds her nose slightly in the air — a habit she picked up after her mother warned her that slouching would give her a hunchback.

Mental Description:
Olivia is quiet and thoughtful, but by no means shy. She enjoys observing more than speaking, but when she does engage in conversation, she comes alive. She makes friends easily, though her relationships often remain on the surface — she connects quickly, but not always deeply.
She tends to make decisions carefully, but her thinking is more emotional than logical. This means that even well-considered actions can appear impulsive from the outside.
Like most people, Olivia fears the unknown — not necessarily strangers, but unfamiliar places and situations. One of her strengths is her ability to detect dishonesty quickly, which makes honesty in others something she deeply values.
Her sense of humour is a bit dry and at times even cynical, though she enjoys a good laugh. Olivia loves music and has tried many instruments over the years — piano, flute, guitar, violin — but she’s never really stuck with any of them or become particularly skilled. She just enjoys the experience of trying.

Biography:
Olivia’s mother, Estelle O’Brian (née Featherstone), is a Muggle-born witch. After finishing her education at Hogwarts, she chose to live a mostly Muggle life — a choice that became even more solid when she met David O’Brian, a Muggle doctor and Olivia’s father. Estelle rarely uses magic, reserving it mostly for practical things like cleaning.
David works long hours at a hospital, so Olivia doesn’t see much of him during the week. Estelle is a struggling author who works from home and is often frustrated and exhausted. She’s strict with her children and quick to scold, but it’s clear she loves them deeply. Olivia, still young and somewhat oblivious to the emotional weight around her, generally gets along with her mother and isn’t yet too bothered by her moods.
Olivia has a brother, Billy, who is six years older and non-magical. David takes the children on weekend trips whenever he can — to parks, other towns, or for film marathons at home. Billy suspects their father’s enthusiasm for travel has more to do with escaping the tension between the parents than with family bonding.
The family lives in a large house on the outskirts of Bodmin and keeps a distant relationship with their neighbours — largely due to Estelle’s unpredictable moods.
Olivia attended a regular Muggle primary school, where she was never particularly outstanding. She struggled especially with maths, though she managed. She’s travelled quite a lot thanks to her father, though she doesn’t actually enjoy travelling much herself. Planes, trains, boats — none of it appeals to her. If it were up to Olivia, she’d simply stay at home, even if that means dealing with her mother’s sharp tongue.

First Instance of Magic:
For a long time, her parents thought Olivia might be non-magical like her brother. But everything changed shortly after her tenth birthday.

One evening, Olivia had a heated argument with her father. He had once again broken a promise to spend the evening with the family and was already halfway out the door. Frustrated and hurt, Olivia shouted after him. “You always do this“, she said. Her voice shook more than she wanted it to. “You never stay.“ She could see that her father was sorry, and yet he still stepped out the door to leave her behind once again. In that exact moment, the front door slammed shut with a loud bang — though no one had touched it.

Olivia stared at the door, her heart pounding. She hadn’t moved. She hadn’t even thought about doing anything. But her breath came quick and shallow, and something inside her felt hot and tight, like a scream that had been trapped for too long.

Her mother stood frozen behind her.

Olivia remained in the hallway, wide-eyed, her fists clenched at her sides.

That was the first unmistakable sign of magic.

Not long after, her parents sat her down in the kitchen. Her cocoa went cold in her hands as they told her the truth: she had inherited Estelle’s magical abilities — she was a witch.
Olivia didn’t cry. She just looked up and quietly asked, “Do I have to go to Hogwarts?“
She wasn’t thrilled by the idea of being sent away to some strange, far-off castle but Estelle was firm. Her daughter needed proper training — something she couldn’t give her on her own.

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[Since English is not my native language, I translated this with the help of Google.]