A Missouri woman who was killed days after Christmas, along with her children and mother, appeared to have a "wonderful" relationship with the boyfriend now charged in their deaths, said her grieving father.

Kate Kasten, 39, her daughter Zoe, 8, son, Jonathan, 10, and mother, Jane Moeckel, 61, were shot just before midnight Friday in Kasten's home in St. Charles, Missouri, a suburb of St. Louis.

Richard Darren Emery St. Charles Police Department

With his voice breaking, Kasten's father, Rick Moeckel, told NBC News that he was in disbelief that the boyfriend, Richard Darren Emery, who goes by Darren, could possibly have committed such a horrific crime.

"She had been dating him for a year, a year and a half. I thought I knew him well," Moeckel said. "[But] the booking picture shows a Darren Emery that I did not know. I looked at that picture and it just looks like there’s no soul there. That’s not him. You just can't fathom or understand what the heck went wrong in his brain that caused this."

On Monday, Emery was arraigned on 15 counts, including first-degree murder, assault and attempted robbery. He appeared in court still wearing bandages from injuries he sustained during a shootout with police during his capture, and did not have an attorney, reported NBC affiliate KSDK in St. Louis.

Emery did not enter a plea and is being held without bond. He could face the death penalty, prosecutors have said.

Kasten had met Emery at a concert not long after her husband Kory died following a long struggle with cancer. He seemed to make her happy, Moeckel said.

Richard Darren Emery and Kate Kasten via Facebook

"He filled a need that helped her get through things, and there was never any indication that something like this could happen," Moeckel said. "He was maybe a little high-strung, but he never raised his voice. I never saw him get mad."

The couple always smiled when they were together, Moeckel said. Emery, a truck driver who delivered tires, frequently described Kasten, a salesperson for a lighting company, as "the love of his life," Moeckel added.

Jonathan Kasten, 10, and Zoe Kasten, 8. Kate Kasten / via Facebook

The two lived together along with Kasten's kids. Kasten's mother, Moeckel's ex-wife Jane, was also living in the house at the time of the killings: Four days prior, she had had hip surgery, and had planned to live there while she recovered.

Moeckel, who lives 15 minutes away, would see his grandkids "all the time," and often spent time with Emery at their house. Emery was friendly, but at times insecure, he said, recalling a visit a couple days before Christmas when he and other family members went over for dinner.

"He was deep-frying a turkey, and he loved people to tell him how great he was doing. He needed people to tell him that you're a good guy," Moeckel said.

Moeckel said he had no idea what could have led Emery to allegedly commit the quadruple homicide. An investigation is ongoing; authorities have also said they have not found a motive.

“We may never know,” St. Charles County Prosecuting Attorney Tim Lohmar said at a news conference Saturday.

Authorities first got word of the killings as they were unfolding Friday night, according to the St. Louis Post-Dispatch, which reported that Jane Moeckel called 911 from a room downstairs, where she had barricaded herself with her two grandchildren.

Gunshots rang out in the background of the call, the paper reported, and by the time police arrived, Jane and the kids were already dead. Kasten, who was discovered in critical condition in the master bedroom upstairs, later died from her injuries at the hospital.

Police say Emery, 46, was fleeing the house in his pickup truck when they arrived. After exchanging gunfire with them, he escaped, at one point stabbing a woman who he had tried to carjack multiple times, they said. The woman is in critical condition but expected to survive.

About seven hours later, authorities found Emery hiding out in a gas station bathroom, bleeding from gunshot wounds that he likely sustained during the police shootout.

Emery does not appear to have any criminal history. Missouri records indicate he has been divorced twice.

A vigil was held for the family on Sunday night. Moeckel said he estimated it drew a crowd of up to 400 people — not surprising, given how friendly Kasten and her kids were.

"She was probably the most vibrant, outgoing person, along with her kids. They have hundreds of friends," he said.

Zoe was a talented athlete, Moeckel said, who started playing field hockey at 6 years old and was planning to try out for a lacrosse team next year. Jonathan could be quiet at times but had a good sense of humor and loved to make people laugh.

"He seemed to do everything he could to make life good for them."

The kids appeared to adjust well to having Emery in their lives following their father's death: "I’m sure it was difficult, but he seemed to do everything he could to make life good for them," Moeckel said.

Since the killings, Moeckel said he has been getting through his grief "moment by moment."

"Lots of tears," he said, "but we're trying to keep it together."