I’ve spent ten months in the presence of Lucy Letby, and I still don’t understand her. I’m not sure what you’d expect Britain’s most prolific child killer to look like. But I’m pretty sure it’s not this.

Photos on social media chart Letby’s old life – nights out with friends, dressed up and goofing about for the camera. She doesn’t look like that now – her dyed blonde hair has returned to its natural brown.

Behind the glass screen of the dock she cut a feeble figure, flanked by prison officers and clutching a pink scarf like a comforter. A severe expression replaced the smiles from her photos.

The families of the murdered babies filled the public gallery. Across the aisle, most of the seats were empty. But the nurse’s mother and father, Susan and John, showed up, day after day, and they were sometimes joined by one of their daughter’s friends – the only one to come.

My berth on the press bench was no more than five metres away from Letby’s seat. Every so often I’d look across at the nurse, to try and catch a glimpse of character. As bereaved parents recounted the horrors of watching their children die, the nurse maintained a neutral expression. No matter how emotionally charged the evidence was, she sat passively.

Very rarely, as she was brought in and out, she’d look up and catch my eye, but just as quickly, she’d look away again. I tried to look into her soul. I drew a blank. I started to question whether we’d ever see the real Lucy Letby.

The trial began in October and as the court broke up for the holidays, I wondered what sort of Christmas she was having, behind bars in prison in Yorkshire.

It wasn’t until February that I first saw a hint of emotion from Letby. It wasn’t prompted by an upsetting piece of evidence, or harrowing testimony. It was the voice of a doctor that caused the nurse to break.

She couldn’t see him – he was hidden behind screens to protect his identity – but she could hear him speak, and his voice seemed to trigger feelings we hadn’t seen before.

Later, Letby admitted she had “loved him like a friend”. We were shown flirty texts between the two, which suggested that although the doctor was married, it might have been more than that. The prosecution painted him as her boyfriend.

I found it interesting that while the nurse remained composed throughout months of evidence relating to the terrible suffering of tiny babies, her first sign of emotion seemed to be borne out of pangs of longing for this doctor.

There were only a handful of other occasions when tears came to the surface. During evidence about being taken off nursing duty, when excerpts of her post-arrest interviews were read out, and when it was mentioned she’d had suicidal thoughts.

Much later, when lead prosecutor Nick Johnson KC got to his feet to start cross-examining Letby, his first question was one I’d been wondering too.

“Is there any reason that you cry when you talk about yourself,” he asked, “but you don’t cry when talking about these dead and seriously injured children?”

“I have cried when talking about some of those babies,” Letby replied.

The first buds of spring arrived, and the trial trundled on.

The dense evidence was hard going. Blood gas records. Fluid balance charts. Clinical notes. The glossary of medical terms handed to the media at the start of the trial had become redundant. By now we were all fluent in the terminology of neonatal medicine.

The prosecution’s case was carefully built on data and documentation, but it wasn’t evidence that gave any clue about Letby’s character. As the case progressed without any insight into her possible motives, the nurse’s personality remained the elephant in the room.

Occasionally, something would cast a shard of light on Letby’s life. The jury saw photos of her house taken by police after her arrest. Art covered in clichéd quotes hung on the walls. A Dream is a Wish Your Heart Makes. Sparkles Wherever You Go. Shine Bright Like A Diamond.

There were teddy bears on the bed. Artificial flowers. A fluffy pink dressing gown hanging on the back of her bedroom door. Monopoly and Trivial Pursuit. A Mrs Doubtfire DVD.

Two books sat by Letby’s bedside. In Shock, a doctor’s memoir about being dangerously ill after a miscarriage, and Never Greener, a novel about a young woman who had an affair with a married man.

In the autumn, the case had opened with a flourish when the prosecution produced a green post-it note discovered by police after Letby’s arrest. Covered in a desperate scrawl, it included phrases like, I AM EVIL I DID THIS, I killed them on purpose because I’m not good enough, I don’t deserve to live, I am an awful person.

The prosecution held it up as a confession. The defence argued it was an anguished cri de coeur written by the wrongly accused.

Either way, it was the most significant insight we had into Letby’s state of mind. I wrote to the judge to ask for permission to make it public. He agreed.

Several months on, the trial returned to the note. It turned out it wasn’t the only scribbled memo police had found – Letby had covered all sorts of pieces of paper with her ramblings. Tightly packed lines of handwriting laid bare her mindset as she was taken off duty as a nurse and the net closed in.

Please help me, I can’t do this any more, Hate my life, I want someone to help me but they can’t were all scrawled alongside the names of friends, colleagues and the married doctor, whose name was embellished with love heart doodles.

The names of her cats, Tigger and Smudge, appeared frequently.

One of the notes was found inside Letby’s 2016 diary, a journal with a cartoon bear on its cover and the tagline, “Have a lovely year!”

We were shown a week in which she’d noted a reminder to pay her council tax, and diarised a night out at a Mexican restaurant and a salsa class. This was the same week she murdered two brothers. The baby boys were triplets.

I tried to get my head around the possibility of this double life.

WhatsApp and Facebook messages Letby had sent to friends and colleagues were shown to the court every so often, but it was hard to build up a picture of the nurse’s character through individual texts.

I spent time compiling them and started to spot some interesting themes. Quite often she’d text other nurses to tell them about her involvement with babies who had collapsed – it looked like she was fishing for sympathy.

Certain messages hinted at a possible God complex.

Other texts sent a chill down my spine – including one written the night before she returned to work after a holiday.

And one she sent about two brothers.

It was particularly fascinating to read Letby’s texts as she began to realise she was under suspicion.

We were deep into the prosecution case, and I still couldn’t marry up Letby’s apparent normality with the enormity of the allegations she was facing – but the case against her was beginning to stack up.

Then I met Dawn.

Dawn didn’t feature in the trial, but she and Letby go way back – they grew up together and are still in touch.

Dawn was immediately warm and likeable. We went for a drive and she pointed out the cathedral green where she and Letby used to hang out, and their favourite restaurants.

“That’s where we used to spend lunch times, away from all the popular kids,” Dawn told me as we drove past the geography block of their old school.

“You weren’t popular?” I asked.

She laughed. “No, we were the nerdy ones that concentrated on our studies, and didn’t mess around in the lessons.”

The friends had moved on to sixth form college together, and while most of their circle had no firm career plans, Dawn told me Letby was clear about her path.

“It was always her aspiration – her dream – to become a nurse and to help babies,” Dawn said. “She told me she’d had quite a difficult birth herself and was quite poorly, and I think that’s affected a lot of her life.

“She feels that’s what she was called to do – to help children who might have been born in similar circumstances.”

Unwavering in her loyalty and belief that her friend was incapable of murder, was it possible that Letby had pulled the wool over her eyes?

Dawn let out a long sigh, before answering.

“The only way I’d ever believe that she’s guilty is if she tells me she’s guilty,” she said.

I was struck by Dawn’s certainty, but my own mind was far less settled. Like Dawn, I needed to hear directly from the nurse herself.

Professor David Wilson, a criminologist with an interest in healthcare serial killers, told me Letby was facing a “crucial decision” about whether to give evidence at the trial – or not.

“I’ve seen people do it and they unravel within the first five minutes,” he said. “They might be clever, they might actually hold their own, but their entire attitude in the witness box can really prejudice what the jury thinks about them.”

Professor Wilson said the outcome of the entire case might hinge on whether or not Letby decided to take the stand herself – which she finally did, at the start of May.

I came into court one morning, and Letby was sitting just in front of me, staring straight ahead. She looked tense and kept her hands clasped below the counter.

She was asked to stand, gave her name, and swore to tell the truth. I was gripped.

The nurse’s defence barrister, Ben Myers KC, got to his feet. He started gently, with questions about Letby’s childhood and school days – benign stuff, but I hung on every word – after seven months it was captivating just to hear her speak.

Letby came across as well-spoken and unflustered, thoughtful and co-operative.

I started to detect certain phrases she had on repeat. Asked about the Facebook searches she made for the babies’ parents she replied: “That was a normal pattern of behaviour for me.”

And asked about taking nursing documents home with her, and storing them? “That was a normal pattern of behaviour for me,” she said. It sounded rehearsed.

After five days of relatively tame questioning from her own barrister, the prosecutor, Nick Johnson KC, bore down on Letby. The easy ride was over.

What followed was the court at its most compelling. At first, Letby coped well. She clearly felt equal to her interrogator, and her knowledge of neonatal medicine was obvious – sometimes it veered on cocky.

She disagreed with established nursing guidelines, senior doctors, and medical experts. There were even moments when she tried to outsmart Johnson. Those never ended well.

The prosecutor picked holes in her testimony, pointing out the differences between what she’d told the police after her arrest, and what she was saying in court. He found examples of her disagreeing with herself – highlighting evidence she had previously agreed and was now disputing.

“You’re lying aren’t you, Lucy Letby?” he’d ask her. “You enjoyed what was going on didn’t you, Lucy Letby?”

“No,” she’d answer, meekly. It was clear he was getting to her.

The defendant’s delivery started to change. She became staccato and monosyllabic. Her voice level dropped to a whisper, and even though I was just a few metres away, it was becoming harder and harder to hear her.

And then, for the first time, Letby asked to stop.

Nick Johnson had been asking her about each baby in the order they appeared on the charge sheet. We were only four babies in – I remember wondering how on earth she was going to manage to get through the remaining 13.

The jury was asked to leave the room, and we were told Letby’s welfare officer had visited her. The court finished early for the day and the prosecution team walked out looking jubilant.

They had her on the ropes.

In total, Letby spent 14 days in the witness box and faced nearly 60 hours of questioning – but did I feel any clearer about her true self? No.

She returned to the glass walled dock for the rest of the trial. June turned to July. The lawyers closed their cases, and the judge summed up the evidence.

Now the nurse’s future was in the jury’s hands. They had nine months of evidence, and 22 charges to work through. Was Letby evil personified, or a victim herself? How they felt about her would determine the rest of her life.

Finally, the answer.

The smiling nurse with the sing-song name who went to salsa classes is now Britain’s most prolific child murderer. Can anyone make sense of that? I know I can’t.

If you, or someone you know, need help after reading this story, details of organisations offering assistance can be found on the BBC Action Line website.