A mental health trust has admitted its failure in the care of a 16-year-old inpatient led to her death.
Elise Sebastian, a Harry Potter fan and music lover, was found unresponsive in her room at the St Aubyn Centre unit in Colchester in April 2021.
Staff at the unit, run by the Essex Partnership University NHS Foundation Trust (EPUT), had been tasked with providing one-to-one care.
The family attended the first day of an inquest into her death and they are participating in the ongoing Lampard public inquiry which is investigating more than 2,000 patient deaths.
The Lampard Inquiry team will monitor Ms Sebastian's inquest.
The inquest at Essex Coroner's Court heard how Elise was supposed to be on one-to-one care at the St Aubyn's unit after several self-harm attempts.
Essex area coroner Sonia Hayes said she was supposed to be observed when she was in isolated areas.
Mother Victoria Sebastian visited Elise on 17 April that year and gave her a cuddle.
But, when she left, Elise was allowed to enter her bedroom from a communal area without staff members present.
The teenager, who lived in Southminster near Maldon, was found unresponsive and died in hospital two days later.
Mrs Sebastian told the inquest that the pain of her death was "too much to bear" and Elise was "badly let down by the system".
EPUT accepted the trust's failures were "causative of her death" and its lawyer Pravin Fernando said: "[It] failed in its responsibility by allowing her to enter her bedroom unsupervised."
The family also said EPUT staff had not told them where Elise was after she had already been rushed to hospital – so they drove to Colchester Hospital in the hope she was there.
Michael Lewis, a senior paramedic for the East of England Ambulance Service NHS Trust, was called after Elise was found unresponsive.
He said he was met with several locked doors and became "extremely frustrated".
"I felt there was no-one there taking responsibility," he said.
"Each question was met with 'I don't know'."
He continued: "I was told the patient was on one-to-one observation so I remember thinking – how has this happened."
Dr Lisa Cunningham, a consultant in emergency medicine at the Essex & Herts Air Ambulance, said in a statement there was an "air of stress from the staff".
Mrs Sebastian said family meant everything to Elise, and she had a fantastic relationship with her brother and two sisters.
Her father Glen Sebastian described her as a "typical little princess" and said they would enjoy visiting places like the West End and art galleries.
"She would blow me away with her paintings and she certainly had a gift," he said.
Ms Hayes heard how Elise was diagnosed with autism and sometimes struggled to understand other people's behaviour.
She had physical health issues including bowel problems and a curvature of the spine.
The teen was also diagnosed with epilepsy which affected her confidence socially.
The coroner was told she first struggled with anxiety aged 12 and had several admissions to mental health units when she was 15 after being bullied.
She managed to abscond from the unit on previous occasions.
Both Mr and Mrs Sebastian, who are now separated, said they had suffered with post traumatic stress disorder since her death.
In her opening statement, Ms Hayes said Elise was: "A young person with her own hopes and dreams."
The inquest is expected to last for four weeks.