She's the nanny who says she was Jon Gosselin's lover, and she's not pulling any punches.

"He's a liar," says 24-year-old Stephanie Santoro, who tells INSIDE EDITION the TV dad promised her a life of wealth and comfort, and then dumped her. "I'm just left thinking why? Why could someone lead someone on so much and then lie so much in the end."

Stephanie says she met the smooth-talking Jon Gosselin at a Pennsylvania bar where she worked as a waitress. "He came in and we were standing there talking about he needed another full-time nanny and asked me if I would want to come do that."

She says he invited her over to his house to talk it over when Kate was out of town.

"We were sitting by the pool at first and then we were in the Jacuzzi..."

"You brought your bikini or you went in your underwear?" asks INSIDE EDITION's Paul Boyd.

"A bikini, he told me to bring my bikini, that we would relax in the Jacuzzi, sit and talk."
"And what happened?" asks Boyd.
"Something I never thought would ever happen."
"You did sleep with him that night?" asks Boyd.
"I did," she admits.

That night Santoro says she became Jon Gosselin's lover and his nanny. By day she would look after the kids, and by night she and Jon would have sex in his apartment above the garage, where Jon has been sleeping when he's home to watch the kids ever since his split with Kate.

"How many times did you interact sexually?" asks Boyd.
"Nine times."
"And you didn't feel at all like he was using you?"
"He was very genuine; he's very, very good with his words. He would say it's sincere, he's being real, this is how he truly feels," says Santoro.

She says the affair began at the end of July, when Jon and Kate were in the early stages of their bitter divorce battle. Stephanie says Jon sent her hundreds of text messages, which she shared with INSIDE EDITION.

"This one says: 'I'm worried the girls might say something to Kate, I don't want to ruin your chances of working here.' "

"He was worried about Kate finding out about your relationship?"
"Yes," Santoro says. "He just wanted me to be there only when he was there, because Kate didn't like anybody and Kate was mean to everybody, and it's her way or no way."
"Weren't you trying to pull the wool over her eyes?" asks Boyd. "Making it sound like you were something you weren't?"

"Yes and no," says Santoro. "When I was there watching the kids that was my sole purpose and my priority, there was never any type of relationship in front of the children."

But the relationship began to sour when Stephanie says Jon revealed his dark side, she says he told her he was spying on Kate, something Jon has denied.

"He said that every time she would send or receive an e-mail that he would know exactly what it was, that he would get a copy of it, every time she would send or receive a text message that he would know who she was talking to."

"You stand by 100 per cent that Jon hacked into Kate's emails? That's what he told you he was doing?" asks Boyd.

"That's exactly what he was doing, he knows how to do it, he brags about knowing how to do this stuff."

She also claims that Jon smoked marijuana when he was home alone with the kids.

"As far as you know Jon was at home getting high while the kids were in the house?"

Santoro says, "Yeah, he said they were in bed and that he was out back smoking right before we got there."

Jon Gosselin told INSIDE EDITION he would not comment on Santoro's claims, but denies that he had a sexual relationship with his one-time nanny. When told that Santoro has accused him of using drugs while watching the children, he said, "You can drug test me, you can do whatever you want, but I have not used any drugs."

"Jon says you're lying," Boyd tells Santoro.

"I have proof. I have hundreds and hundreds of text messages that can back up everything I say," she says.