The teenager told the jury he first met Willems a month or so earlier when the latter pulled over beside him near Haruru Falls when he was riding his push bike, asked what he was doing, and offered him drinks and cannabis which he accepted.
They spent a few hours together that day and exchanged texts after the first meeting.
On January 7, he said Willems texted him to say he had bought and new car and whether he wanted to go for a ride.
He and a male cousin went with Willems and they bought liquor and cannabis and went to various places.
The teenager said he texted a female cousin during the course of the day and asked if she wanted to also come for a drive and she agreed.
After more drinks and cannabis at a spot near a river, it was decided the female cousin would drive as she was sober.
The 15-year-old said at the McDonalds drive through in Kerikeri, Willems tried to reach for her leg from the front passenger's side which annoyed them.
On another occasion, he said Willems put his arm around her as he was trying to impress his cousin with his car and money.
Willems started talking about drugs, gangs and having been to jail and that upset the cousins more, he said.
At one point between Kawakawa and Moerewa, the defendant said Willems told his male cousin he would kill him.
The cousins and Willems argued on stopovers in central Kawakawa and at the Bay of Islands College carpark before they asked Willems to drop them off where they were staying near the tavern.
On the way home, the teenager said his female cousin had a panic attack, was crying and couldn't breathe as a result of Willem's actions.
When they arrived at the drop-off point, he said Willems got out of the car and wanted to see her but they told him to leave.
He said Willems hid a knife under his sleeve, came round behind the car, asked him if he had a knife and when he replied in the affirmative, Willems said "come on then" and lunged towards him.
The teen had a pocket knife which he always carried for personal use.
Both swung their knives at each other before the male cousin knocked the knife off Willems.
"It was dark, I couldn't see anything. I was swinging and we both ended up on the ground," he said.
Defence lawyer Ron Mansfield will call an expert witness on Monday who will talk about young people's cognitive development and behaviour.