First time offender Bradley King was on remand at Manawatu Prison awaiting sentencing on minor commercial burglary charges.
Just 10 days after he was locked up - his bid for bail denied - he was bashed so badly by his cellmate as he slept that he may not survive.
Today the family spoke about their "beautiful gentle giant" and how a mistake he was so sorry for and wanted desperately to make amends for may cost him his life.
This morning Bridget King was on her way to Wellington Hospital's intensive care unit to see her twin brother before major surgery to relieve pressure and swelling on his badly injured brain.
"It's the last resort," she said.
"But we will never give up on him.
"We're broken, we're hurt, we're so, so angry… we just want to know what happened."
King, 36, was charged with three counts of burglary from commercial sites.
It is the first time he has been in trouble, before the courts or in custody.
He applied for bail but his sister said police opposed that and he was held on remand.
"He was in prison for 10 days and he was terrified, but he made friends and he kept his head down,' Bridget King told the Herald today.
"He was attacked in his sleep, he didn't stand a chance."
An investigation into the attack is ongoing and charges are yet to be laid.
Bridget King said police and Corrections were both looking into the incident and the Corrections Minister Kelvin Davis had been in touch with her family.
The name of another inmate had been published on social media alleging he was the attacker but that was incorrect.
A Corrections spokesperson confirmed both King and his alleged attacker had not been in custody before.
They arrived in prison on the same day.
"The alleged perpetrator has been segregated," the spokesperson said.
"Corrections is carrying out an operational review into the incident, and police are also investigating.
"We appreciate that this is a very distressing situation for them, and staff have visited the hospital to offer them our support.
"We have a zero tolerance for violence policy, and any violence or assaults against other prisoners or staff is not tolerated in prisons. Any prisoner using such behaviour will be held to account for their actions, including facing criminal charges."
Bridget King said her twin brother sustained catastrophic brain and head injuries, multiple skull and facial fractures, brain bleeds, a punctured lung and broken ribs.
"He may also have internal injuries that they have missed at the beginning, but because he is so unstable they cannot scan him," she explained.
"He's in an induced coma… We are living hour by hour with him… we sit by his bedside, we talk to him… we are with him."
She said the family had a lot of questions about what happened, why and the decision around who he was sharing a cell with.
But for now they were focused on King.
"What we want people to know is that he didn't deserve this - he was on remand for three small commercial burglaries, he was going to plead guilty and he was so, so remorseful," she said.
"He comes from a huge, loving family and he is so loved.
"He's our gentle giant, he's never hurt anyone and he would help anyone who needed it… he wouldn't hurt a fly.
"Yes he did small burglaries but he had admitted that, he wanted to say sorry - and before people judge, they need to realise he's not a bad person, he's got a family and friends who love him so much and he had a lot of remorse.
"Now we face the possibility of losing him because he made a mistake… we're devastated.
"If he does survive this, he will live with the injuries for the rest of his life - but we're not giving up on him, we will never give up on him.
"Today is a last resort surgery… it's day five and he's still with us so we can't give up."
Bridget King said hundreds of people had messaged her family to show support since the attack - including some inmates her brother had met in the short time he was jailed.
"That's testament to what an amazing person he is," she said.
"Yes - he made a mistake, he knew that and he wanted to make it right.
"But he hasn't had the opportunity to do that, and ultimately, he might lose his life over this."