Speaking to journalists for the first time since the attack, Tracey Andrews, 27, described the killer as having "starey eyes" and said that he could quite possibly have been on drugs.
"He didn't seem normal," she said. "I can't say he seemed drunk but it seemed to me he wasn't normal as if he had taken something."
Ms Andrews, who was punched by the killer and suffered two black eyes in the assault, described Mr Harvey as "a lovely, kind, generous man" and said the couple had planned to marry next June. "This is the most stupid, vile thing that could come out of just a car chase," she added.
Mr Harvey, of Alvechurch, Worcestershire, was attacked by a passenger in a "tatty", F-registration, dark-coloured Ford Sierra late on Sunday night after the driver had followed his Ford Escort for several miles flashing its lights.
When he got out to confront the driver, the passenger got out and stabbed him15 times in the throat, body and face, leaving him to die on the roadside.
Ms Andrews said the couple had been out for a drink earlier that evening at a pub in Bromsgrove before returning via a country lane to their home in Alvechurch:
"We were just followed and chased along the lanes. It was the case of both Lee and the other person playing cat-and-mouse with each other for a while and then they overtook us. Lee decided to pull in and got out of the car.
"There was some sort of argument going on, just calling each other names and a lot of swearing going on. I don't know what was said but I know it was to do with the driving."
She said a passenger from the car walked up and hit Mr Harvey which prompted her to challenge the man.
"We had a confrontation and he hit me," she said. "When I got up he was walking back to the car and Lee was lying on the floor.
"The driver walked off. It was nothing to do with the driver, and I want to say will the driver of the car come forward because you are not to blame for this and I know that because you walked away.
"But you obviously knew him [the killer]. He has ruined my life and he has ruined the life of Lee's parents. It was not your fault. Please come forward."
She described how she put her coat over her fiancee and comforted him as he lay bleeding on the ground: "I was just in a state of shock myself. I knew it was bad because I saw a lot of blood."
Detective Superintendent Ian Johnston, leading the hunt for the killer said the response to appeals for witnesses had so far been slow although several names had already been put forward for the driver and the attacker.
The killer is described as a white male 5ft 9 inches to 6ft tall, very overweight and wearing a dark thigh-length donkey jacket. He spoke with a Birmingham accent and would have been heavily bloodstained following the attack.Reuse content