The teenager held over the Southport stabbings was an 'introvert' who lived in a nearby village with his 'nice family', who had moved to the UK from Rwanda.
The 17-year-old boy was born in Cardiff after his family moved from the African nation - then moved north to Merseyside about ten years ago.
He remains in custody accused of murder and attempted murder and is being questioned following the incident.
Merseyside Police said today that a third girl, aged nine, had died in the early hours of Tuesday morning. The two girls who died yesterday were aged six and seven, a spokesperson added.
Eight other children were injured with five of them - aged between six and 11 - in a critical condition alongside two female adults who 'bravely tried to protect them'.
The suspect's former neighbours said they were 'shocked' and 'saddened' to learn the teenager had been arrested over the horror knife attack as he comes from such a 'nice family'.
As a youngster he trained in karate with his father doing classes with Sensei Chico Mbakwe, 79, at St Cadoc's church hall in Llanrumney - a suburb of the Welsh capital.
The suspect's former neighbours said they were 'shocked' and 'saddened' to learn the teenager had been arrested over the horror knife attack (Pictured: His childhood home in Cardiff)
Chilling footage has emerged of masked figure pacing outside a house that was later raided by police after the Southport attack
Armed police officers later swooped on the address where an individual had earlier been pacing up and down
Police officers near the scene in Hart Street, Southport, this morning following the stabbing yesterday
A forensics officer is seen walking past the cordon in Hart Street in Southport
A person leaves flowers near the scene in Hart Street, Southport, where the event took place
A little girl is helped as she takes flowers to leave with the line of floral tributes
Mr Mbakwe said the boy was a 'typical, normal five year old' who continued to train at the dojo even after his father fell out with club leaders.
Mr Mbakwe said: 'He was a good child, a normal child, who had lots of energy.
'I remember him because he continued to train with us for around three months after his father left for another dojo I own.'
Mr Mbakwe said the family were nice but the boy's father fell out with him after he failed him for his black belt grading.
He said: 'Later when he moved to Southport he emailed me to tell me that he had finally passed his black belt - and said 'thank you for failing me, I realise now that I wasn't good enough'.
The family of four lived in St Mellons and Thornhill areas of Cardiff before moving to Southport in 2013.
Mr Mbakwe added: 'The dad was just a pleasant, normal guy, and the son was just a normal five-year-old.
'Children don't tend to stand out that much at that age but I do remember him.
Emergency crews and forensic teams descended on the scene after 13 people were stabbed
An aerial view of Hart Street, where a horrifying knife attack took place just before midday
Floral tributes pile up by the side of the road as locals flock to share their sympathy after the tragedy
Southport locals have also rushed to leave their kind words, with one saying their 'thoughts and prayers are with you'
The attack happened at a Taylor Swift-themed workshop on Hart Street in Southport. Police later swooped on a property in Banks, five miles away
'I'm sad to hear what has happened. It makes me worry about my grandchildren and great-grandchildren with all the knife crime around these days.'
The couple lived in the Welsh capital for 11 years and had two boys, the eldest boisterous, the youngest quiet and shy.
Former neighbours were in shock after discovering the youngest boy had been arrested for murder and attempted murder following the 'ferocious' stabbings at a Taylor Swift themed dance class for young girls.
A man living in the couple's former two-bedroomed home said: 'Obviously something has gone seriously wrong with this young man.
'It is a shock, such a terrible thing to happen.'
Their former next door neighbour, a 55-year-old civil servant who didn't want to be named said: 'The youngest child was very quiet, an introvert
He was quite clingy to his mum while his older brother was more boisterous and would stick his tongue out at you.
'I knew the parents were from Rwanda, they spoken English with a strong accent and they cooked spicy food with smells that I didn't recognise.
'They were in their late twenties, they had a story about how they came here but I didn't like to pry.'
'But they were an ordinary couple struggling to make a go of things here. I would speak to them over the garden fence but didn't get to know them well. They were renting here for about 18 months to two years.
'The youngest boy hadn't started school but the older one was at the primary nearby.
'The eldest boy would be kicking footballs, running around screaming but the youngest was quiet, he was very young.'
'The dad went out to work but I think she was a stay-at-home mum. I remember the dad saying he'd got a job in Liverpool and I thought 'good for you mate'. I remember it was quite a rough area. I wished them well when they left.
'They had a car, a small hatchback, I don't think she drove.
'They must have had lots of money, it wouldn't have been a cheap rent.
'This is a lovely estate in a nice part of Cardiff.'
Former neighbours in the suburb of Thornhill, where the family lived in a modern three bedroom semi-detached home said the boy and his younger brother were 'nice enough'.
One woman said the family rented the house in a cul-de-sac when the boys were aged around four and six before their father got a job which took them up to Merseyside.
She said: 'I remember they came from Rwanda with two kids and lived here for six months to a year.
'They seemed nice enough. They said they came from Rwanda. I didn't like to ask a lot of questions about that - you think fair enough, you're here now and it's a quiet life.
'We talked over the fence round the back, the kids went to school.
'They were nice children. Always running around playing football in the garden, that kind of thing.'
She said that the cul-de-sac was a 'really nice street' which was 'peaceful and safe' and that she worried when she heard they were moving.
Emergency services rushed to the scene after reports of a stabbing spree
A police officer cordons off the area after the attack that left three children dead
The attack happened at a Taylor Swift themed yoga and dance workshop
She said: 'They were nice parents - they were good parents. They were a nice family, they were nice kids.
'They were very young when they were here - in their 20s.'
Eight children were injured in the stabbings, alongside the three who died and two adults who remain in a critical condition after attempting to protect the youngsters.
At the scene today, flowers and teddies were left by the police cordon. One message in a 'Thinking Of You' card said 'thoughts and prayers' are with those affected and was signed: 'Love - Southport.' Another said: 'Fly high with angels little ones.'
Prime Minister Sir Keir Starmer could visit the town today. Police have said that, although the motive for the attack is unclear, it is not believed to be terror-related.
Witnesses to the incident spoke of how the alleged knifeman arrived at the dance studio in a taxi and got out without paying.
The masked attacker entered The Hart Space - a community centre at the heart of the Merseyside community - through the door, which had been left open in an attempt to keep cool amid the scorching heat.
It's believed he coincided his arrival with parents picking up their children from the workshop, which was supposed to finish at 12pm.
Home Secretary Yvette Cooper pays tribute to the child victims of the horror knife attack
Ms Cooper visited the town to lay flowers at the police cordon on Hart Street in Southport
Emergency services said the scene was 'horrific' and 'chaotic' as they arrived at the children's club. All the casualties are now being treated in hospitals.
Detectives cordoned off an area in the village of Banks, just outside Southport.
Locals said Matrix police, the armed unit of Merseyside Police, descended on the address shortly after the incident in Southport. It also came shortly after the individual was pacing around the front of the property.
Merseyside Police declared a major incident at the scene. with armed response vehicles, 13 ambulances and the fire service descending on the mass casualty scene.
King Charles said he and Camilla had been 'profoundly shocked' to hear of the incident, while Prince William and Kate described it as a 'horrid and heinous' attack.
The holiday club was advertised as including a dance and yoga workshop themed around Swift as well as bracelet making. A vigil for the victims is planned for tonight.
In a social media post, Taylor Swift shared her horror at the 'loss of life and innocence' at the workshop.
She wrote: 'The horror of yesterday's attack in Southport is washing over me continuously, and I'm just completely in shock.
'The loss of life and innocence, and the horrendous trauma inflicted on everyone who was there, the families, and first responders.
Taylor Swift performs on stage during during 'Taylor Swift | The Eras Tour' at Anfield in Liverpool
Taylor Swift said she is 'completely in shock' after the 'horror' of the Southport knife attack
'These were just little kids at a dance class.
'I am at a complete loss for how to ever convey my sympathies to these families.'
Home Secretary Yvette Cooper visited the town to lay flowers at the police cordon on Hart Street in Southport this morning.
Ms Cooper, Southport MP Patrick Hurley, Merseyside Police Chief Constable Serena Kennedy, Merseyside Fire and Rescue Service's Chief Fire Officer Phil Garrigan and Merseyside police and crime commissioner Emily Spurrell walked to the cordon together, each holding a bouquet of flowers.
The Home Secretary stood for a few moments looking at the tributes before laying her flowers and reading some of the messages left by others.
She and Mr Hurley hugged. She is understood to have met with first responders in the town earlier.
Speaking to the media today, Southport MP Patrick Hurley said: 'The town is in mourning, the town is in shock this morning. The incident that took place yesterday was uniquely horrific.
'Southport's a quiet seaside, coastal town. This has just sent shockwaves through the whole community.
'It's a very small town, Southport, and a lot of families across the area will be impacted directly.
'I think at this stage we need to just reach out, make sure that there's a level of community healing going on and make sure we give voice to the solidarity that people need right now.'
Chief Constable Serena Kennedy, of Merseyside Police, said those injured had all suffered stab wounds in the 'ferocious attack'.
She added: 'We believe that the adults who were injured were bravely trying to protect the children who were being attacked.
'As a mum and a nana, I can't begin to imagine the pain and suffering that the families of the victims are going through at this moment in time.
'I wish to send my heartfelt condolences to them.'