The mistress of admitted triple-murder Chris Watts said she was tricked into believing he was a loving father finalizing a divorce — and had no idea his wife was 15-weeks pregnant until she and their two daughters went missing in August.

He’s a liar,” Nichol Kessinger, 30, told The Denver Post in an interview published Thursday about the man she met through work. “He lied about everything.”

It’s the first time Kessinger spoke out about her relationship with Watts, who pleaded guilty earlier this month to the August murders of his wife of nearly six years, 34-year-old Shan’ann Watts, and their daughters, Bella, 4, and Celeste, 3.

An arrest affidavit revealed Watts, 33, was having an affair with a co-worker, though Kessinger’s identity hadn’t been made public until now. Kessenger did not immediately respond to PEOPLE’s request for comment but a law enforcement source confirms to PEOPLE that she is the woman referenced in the redacted affidavit.

While some have speculated whether their relationship contributed to Watts’ motive for the killings, Kessinger told The Denver Post she “barely knew him.”

RELATED: Why Did Chris Watts Murder His Wife & Kids? D.A. Has ‘Partial Motive’

“We had just met,” Kessinger said. She said the two had started a physical relationship in early July but were taking it slow and never spoke about long-term plans. She never met any of Watts’ family or friends.

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The two saw each other four to five times a week, she said, except for when Watts went to visit family in North Carolina at the end of July. That’s when Watts called to tell Kessinger that his divorce was final and he needed help finding an apartment for him and his daughters.

Watts had told Kessinger that he was in the midst of a mutual divorce and that he and his wife were working out financial details.

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

A source close to Watts tells PEOPLE, “It’s very clear that this wasn’t a healthy marriage for many months before Shan’ann 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.”

RELATED: As Chris Watts’ Parents Defend Him, Murdered Wife’s Family Slams ‘Grotesque and Utterly False Statements’

Earlier this week, Shan’ann Watts’ family issued a statement in response to claims by Watts’ family that the couple had a stormy relationship, describing Shan’ann 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.”

Mistress Contacted Police: ‘I Just Wanted to Help’

Kessinger only began to see red flags in August, when Shan’ann and her two daughters were reported missing. She said Watt seemed casual about their absence and didn’t show any emotion. When police showed up at the Watts’ family home to question him, she was “very confused.”

Reading the news finally opened her eyes to the fact that Watts was still married and his wife was pregnant. She immediately became suspicious. “I thought, ‘If he was able to lie to me and hide something that big, what else was he lying about?’ ” she told The Denver Post.

RELATED: Behind Chris Watts’ Decision to Plead Guilty — ‘Eventually He Acknowledged His Life Was Over’

In response to what she had learned, Kessinger said she began questioning Watts that night and the following day. He remained calm and asserted that he would never hurt his family, but he tried to change the subject. she said.

“It seemed off,” Kessinger said. “It got to a point that he was telling me so many lies that I eventually told him that I did not want to speak to him again until his family was found.”

Suspicious, Kessinger said she called the Weld County Sheriff’s Office on Aug. 15 to tell them what she knew. She met with FBI investigators the next day.  “I just wanted to help,” she said. “With a pregnant woman and two children missing, I was going to do anything that I could.”

She told The Denver Post she has never doubted that Watts killed his family. “I don’t think there is a logical explanation for what he did,” Kessinger said. “It’s a senseless act, and it’s horrific.”

RELATED: Why Family Murderer Chris Watts Decided to Plead Guilty: ‘Eventually He Acknowledged His Life Was Over’

After his arrest, under police questioning, Watts allegedly said he would “tell the truth” after investigators told him they found out he was having an affair with a co-worker, according to the arrest affidavit.

He then admitted he killed Shan’ann after flying “into a rage” when, he claimed, he saw her strangling one of their daughters via a baby monitor after he had told her he wanted a separation, according to the arrest affidavit.

But in court earlier this month, Watts pleaded guilty to nine charges, according to prosecutors: five counts of first-degree murder, one count of unlawful termination of pregnancy and three counts of tampering with a dead body.

In pleading guilty, Watts essentially admitted his earlier explanation was a lie.

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