Nathaniel Pope murdered the fun-loving toddler in June 2018, two months after moving into the boy's mother Alicia Watson's West Bromwich flat.
Kemarni died from massive internal injuries which included more than 25 rib fractures. The desperate child even tried to escape the flat on numerous occasions to no avail and family and friends warned his mother about bruises and marks on his body.
After a trial which lasted from November until April Pope, 32, was found guilty of murder and three counts of child cruelty and Watson, 31, was found guilty of causing or allowing a death of a child and child cruelty.
Today, Pope was handed a life sentence with a minimum term of 24 years with eight years for child cruelty to run concurrently.
Kemarni's mother Watson, who the judge explained had a serious mental illness and was a risk of suicide, was jailed for 11 years.
Sentencing the pair at Birmingham Crown Court Justice Amanda Tipples said: "Kemarni was a happy child. He was just under three and a half years old when he was murdered by you, Nathaniel Pope.
"You, Nathaniel Pope brutally assaulted Kemarni in his own home, and knowing he was in severe pain you left him to bleed to death on the settee."
Justice Tipples also outlined the numerous injuries on Kemarni's body found by medical experts after his death, suggesting the boy had been attacked over a sustained period before his death. Justice Tipples also described the pair as "habitual drug users" due to their regular cannabis consumption.
She said: "Alicia, you thought Nathaniel Pope was giving you more support in the flat than you had ever before. I'm sure you had a strong bond with Pope. I am sure together you were abusing Kemarni.
"You, Nathaniel caused the rib fractures to Kemarni. And you did this by kicking and punching Kemarni. The final attack was a sustained attack and would have caused him extreme pain.
"You, Alicia knew Pope was hurting Kemarni and you did nothing to stop it."
Pope sat emotionless in the dock as the sentences were read out whereas Watson, who had sent a letter to the judge yesterday disputing the verdicts against her, rocked back and forth. There were gasps of "yes" when Pope was given a life sentence.
The detective in charge of the investigation explained the difficulty of bringing both to justice.
Detective Inspector James Mahon said: "This was a very complicated investigation because of the two defendants and how they refused to comment from the start, and then in the trial we were obviously worried they would blame each other.
"Pope was apathetic from the start, he tried to wash his hands of the whole thing, he was apathetic on the day Kemarni died and even said he was not in a relationship with Alicia and was only visiting the house.
"We had to rely on the medical evidence. The injuries Kemarni sustained were shocking, his rib cage could have only been broken by someone stamping on him with massive force, we believe Pope did that.
"This is a dangerous man who is off our streets now, he previously attacked a mother on a bus in London in front of her children.
"What upset all of us on the case was the CCTV seeing Kemarni returning to the flat, where he should have been safe. Instead he was murdered."
He added: "The jury need commending for doing their duty in what was a very drawn out case for a variety of reasons, it is because of them Kemarni got justice."
Pope, of Evans Street, Wolverhampton, had previous convictions for drug dealing and battering a woman on a bus in London in 2011 in front of her children. When arrested for the murder of Kemarni he was found to have a rock of crack between his buttocks. The trial heard how both were frequent cannabis users.
Watson, of Raglan Road, Handsworth, had no previous convictions and was described as "suffering from post natal depression" in the run up to the murder. The couple remained together for almost a year after Kemarni's death.
Sandwell Council are undertaking a serious case review concerning the death of Kemarni, though there were no safeguarding issues raised with the council before his death.
Lesley Hagger, Chair of the Sandwell Children’s Safeguarding Partnership, said: “This is a deeply upsetting case. Kemarni was a victim of horrifying abuse. He was ultimately murdered while in the care of people he should have been able to trust to protect him.
"Our thoughts and sympathies are with all those who loved Kemarni. As a partnership, we continuously review how agencies work together to protect children and support families.
"Following Kemarni’s death, an independent review was commissioned, in line with the national Working Together 2015 guidance by the then Sandwell Safeguarding Children Board. The review looked carefully at the role of any services that Kemarni and his family had contact with and shared learning with professionals and agencies.
"There were no identified safeguarding concerns regarding Kemarni prior to his death in 2018, and there was no children’s social care involvement with him. But it’s vital that sad cases like these are thoroughly looked at and, where there are areas for improvement, that any necessary changes are made.
"The review report will now be updated to reflect the conclusion of the criminal proceedings and we will look to publish it as soon as possible."
Giovanni D’Alessandro of the Crown Prosecution Service said: “This was a deeply disturbing case of two people who were supposed to be protecting a defenceless three-year-old boy but instead subjected him to cruelty and repeated bouts of violence. It’s incomprehensible that little Kemarni was subjected to such heinous acts of brutality in his final days, causing him intense and ongoing pain and distress before ultimately killing him.
"Rather than take responsibility for their actions, both Watson and Pope repeatedly lied and sought only to blame one another for all of these crimes.
“My thoughts are with the remaining members of Kemarni’s family. Nothing can bring him back, nor do I imagine that today’s sentencing will bring comfort to those that knew and loved him. What I do hope is that the fact that his killer has been brought to justice and received a lengthy prison sentence will assist them in coping with their grief and loss.”
An NSPCC spokesperson said: “Kemarni Watson Darby’s short life was brutally ended by the cruelty and violence inflicted upon him in his own home by a man who should have been protecting and nurturing him. His mother, at the very least, devastatingly failed to protect him.
“Our hearts go out to those who loved and cared for this little boy during his life. It is vital that any learnings from the Child Safeguarding Practice Review are wholly taken onboard by those involved in children’s lives, so we can all work together to prevent such tragedies happening in the future.”
Anyone who has concerns for the wellbeing of a child should immediately contact the relevant authorities or the NSPCC helpline on 0808 800 5000.