Businessman Warren Lahoud dated Reeva for nearly five years and had met up with his ex for coffee and a chat just two days before she died.
For him, 29-year-old model Reeva — who was fatally shot by Pistorius on Valentine's Day in his Pretoria mansion — was the love of his life and the woman he hoped to marry.
And in an exclusive interview with The Sun, South African Warren reveals Reeva had walked out on him without any warning six months before she was killed.
He did not know why she left and now he never will. But he believed it was only a matter of time before they got back together.
And he fears their last meeting could have triggered blind jealousy in the Paralympian known as Blade Runner — who insists he thought Reeva was an intruder when he fired four shots through the locked bathroom door.
Warren, 27, says: "The last time I saw Reeva was on February 12, two days before she was killed. We met at a coffee shop alone.
"We didn't discuss her relation-ship with him. Neither of us would have wanted to hurt each other by being too personal.
"But it was warm and friendly. It was the sixth month we'd been apart and the talk was how we were, how's business and the family. It was so nice to see her again.
"When she ended our relation-ship we didn't speak for two months then gradually started making contact. Reeva was a big part of my life."
Recalling the day Reeva moved out of the home they shared for four years, he says: "Out of the blue, she said it was over.
"There was no warning and she gave me no clues. It was very strange and I don't even know why now. I was in total shock, I couldn't believe it.
Reeva and I understood, loved and supported each other. It was such an easy-going, fun, loving, relationship. We were so happy together. She made our home warm and loving. That's who Reeva was — our home emulated her.
"I left everything with her. I was struggling to understand what had happened.
"Reeva was my best friend and partner in everyway. No one could ever replace her.
"We supported each other, advised and leant on each other, we lived together for four years and were together nearly five.
"We were definitely going to take our relationship to another level. It was a question of time and she knew that.
"I can't say whether she'd have come back to me, I didn't push anything. Reeva needed to carry on with her life and I needed to carry on with mine, but I wouldn't say I'd given up hope I accepted the situation and let it go.
"There's that saying. 'If you love something let it go, and if it comes back to you it's yours forever'. It wouldn't have helped our situation if I'd gone after her. Maybe we just needed a small break.
"I've got a big business that needed attention and if you don't give your relationship the right care things start to crack and that's what happened.
"But I can't pinpoint the proper reason it ended."
Today, Oscar, 26 — who was the first double amputee track athlete to compete in the Olympics when he ran at London 2012 — will appear before South African magistrates for a pre-trial hearing.
It will be his first appearance in court since he was released on £73,000 bail following his arrest over Reeva's death. He has been staying with his uncle Arnold.
Shocking pictures of his blood-spattered bathroom in the aftermath of the shooting were leaked last week He says he mistook Reeva for a burglar.
The prosecution will argue that he shot her intentionally after an argument.
It has been claimed Reeva had been texting an ex-boyfriend and took her phone into the bathroom.
Others have alleged that Pistorius had texted another woman.
Warren was still coming to terms with their split when he got the devastating news Reeva was dead.
He refuses to utter Oscar's name and is not sure he will even attend the trial, which is expected to take place later this year or early next.
He says: "When I got the call that she'd been shot, my mind couldn't process it. It was the second most hurtful, inexplicable, eruption of emotion I've ever had, besides the funeral.
"There is no part of me that will ever understand this. I have guns, yes, but I don't carry them or flash them around. I respect them, put it that way.
"I don't want to talk about him at all. I have nothing to say about him or to him.
"It's so hard for me thinking, 'If only Reeva had stayed with me she would still be alive — if only I hadn't let her go'
"But that isn't going to bring her back and just leaves me with more pain than I'm already feeling.
"I've got to get through this so I allow the fear and hurt to grip me for a time then let it go.
"It does recur and that's why I'm so up and down. It's been tough, emotionally and mentally. But I can't keep ripping open the wound. I have to allow myself to heal and come back to a level of sanity and emotional stability.
"I've got a strong family and very good friends. My family are also totally ripped up. Reeva was a daughter to them.
To know what's happened to someone so special. who I loved so much, is mad, unbelievable, my wildest nightmare come true.
"I'm a strong-minded person. but I do have my breakdowns and have to get myself through things.
"Don't get me wrong, the hurt is massive but for everyone's sake I just suppress it and carry on.
"But things are getting worse for me. Over the past month or so, the realisationt is setting in properly.
"The shock's wearing off and the reality is hitting me."
Hougaard, 25, was a close friend and ex-boyfriend of Reeva Steenkamp's. Unconfirmed reports say he exchanged text messages with Reeva on the night she died.
Also on the prosecution witness list, according to Africa's eNCA news channel, is Pistorius's former lover Samatha Taylor.
Prior to Reeva's death, the blonde model claimed: "Oscar is certainly not what people think he is."
After Reeva's death, Samantha's mum wrote on Facebook:
"I am so glad that Sammy is safe and out of the clutches of that man."
Also on the witness list are a dozen of Pistorius's neighbours from the high-security Silver Woods housing estate. The prosecution has revealed in court that some of them heard loud shouting coming from Pistorius's home on the night Reeva died.
At the weekend, Pistorius's family issued a statement in which they said he is seeking solace in his Christian faith.
The athlete is charged with pre-meditated murder but claims Reeva's death was accidental.