Chris Watts had an affair with Nichol Kessinger, who he met through work
Mistress of Chris Watts speaks out
Credit: RJ Sangosti/The Denver Post/Getty; Inset: Weld County Sheriff's Office

A woman who was having an affair with triple-murderer Chris Watts didn’t realize he was cheating on his wife with her until his many lies were revealed after his family went missing, she said this week.

The woman, Nichol Kessinger, 30, spoke out to The Denver Post in an interview published Thursday. Kessinger was the woman named in a redacted affidavit for Watts’ arrest which said he was having an affair, a law enforcement source confirms to PEOPLE.

Kessinger said she met Watts, 33, at work in late June, noticing he was attractive and that he didn’t wear a ring. He told her he was separated and was finalizing what he described as a mutual divorce, she said, adding, “I believed him.”

She also said Watts — who pleaded guilty last week to the August murders of his pregnant wife, Shanann Watts, 34, and their daughters, Bella, 4, and Celeste, 3 — “was very soft-spoken. He appeared to be a good listener.”

In early July, the pair began a physical relationship, seeing each other four to five times a week, she told the paper. She said she told Watts she wanted to take things slowly and that he should focus on helping his daughters during the divorce.

The Watts family

“He made me believe that he was doing all of the things that a rational man and good father would do,” she said.

But Watts’ nice-guy facade crumbled after Shanann and the girls went missing and Kessinger learned Shanann was 15 weeks pregnant.

“I thought, ‘If he was able to lie to me and hide something that big, what else was he lying about?’ ” she told the paper.

Kessinger noted that Watts seemed unperturbed about his missing family — “It seemed off,” she said — and she soon cut off contact with him. Two days after the trio went missing, Kessinger called the Weld County Sheriff’s Office and told them and then FBI investigators about her affair with him and his lies.

“I just wanted to help,” she said. “With a pregnant woman and two children missing, I was going to do anything that I could.”

Christopher Watts court case, Greeley, Colorado, USA - 16 Aug 2018
Chris Watts
| Credit: Joshua Polson/AP/REX/Shutterstock

• Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter.

That night, Watts was arrested for the murders.

A source close to Watts tells PEOPLE, “It’s very clear that this wasn’t a healthy marriage for many months before Shanann and the girls died. He had checked out, and from what he says, she had checked out, too. Nichol Kessinger showed interest, and in his mind he was single, so it happened. He really regrets that Nichol got pulled into this.”

Shanann Watts

Earlier this week, Shanann Watts’ family issued a statement in response to claims by Watts’ family that the couple had a stormy relationship, describing Shanann as “a faithful wife, and the most gentle and loving mother in the world to her children Bella, Celeste, and Niko [her unborn child],” according to local TV station KMGH.

The family slammed Watts’ parents claims as “vicious, grotesque and utterly false.”

Watts will be formally sentenced to life in prison on Monday.