(ABC News) -- Brynne, Tess and Savannah from Phoenix are black belts in karate, expert horseback riders and avid musical theater fans. And they perform exorcisms.
"We're just normal girls who do something extraordinary for God," Brynne said. "After seeing an actual exorcism in person, led by us, you will walk away with no doubt, whatsoever."
Brynne, 17, is the leader of the pack, the one the others call the "enforcer." She is home-schooled and a regular on the beauty pageant circuit. Savannah, 20, is known as the "compassionate one," a college student who likes to shop. Finally, there's Tess, "the middle man" because the others say this 17-year-old can play both good and bad cop. She also performs in local musicals.
"There is a war going on every day, being waged against us," Brynne said. "Satan hates us. We know how the enemy is, we know what he's attacking and we can fight back."
Their teacher is Brynne's father, the Rev. Bob Larson, who says he has performed more than 10,000 exorcisms in the last 30 years.
Watch the full story on "Nightline" Tonight at 11:35 p.m. ET/PT.
Before agreeing to perform an exorcism, Larson interviews his clients to determine whether they are, in his opinion, demonically possessed. The client must fill out a questionnaire and give some background on his or her personal history.
But Larson claims that 50 percent of the population is probably affected by demons in some way and his girls are the front line of defense. Armed with crosses, Bibles and holy water, the girls summon the demon within the subject, and then the demon apparently takes over the person's body. Brynne said she can tell when someone is demonically possessed when she looks into his eyes.
"When you look at that person, you could just see the evil looking back," she said.
The girls say there are many different types of demons, each with their own names and personalities. One demon, Brynne said, is named Jezebel and is very proud and haughty.
"There's Hate, Murder, Anger, all of those are very violent demons," she added.
"When a demon comes into someone, it's going to bring as many of its kind with it as it possibly can because its desire is to steal, kill and destroy that person's identity, that person's life," Tess said.
Classic signs of possession, the girls said, include when a possessed person starts speaking in tongues, reciting historical facts he wouldn't know otherwise, or having superhuman strength.
Performing exorcisms can be dangerous work, and Larson's wife said she was reluctant to let their daughter, Brynne, do it. But Bob Larson believes it is a good lesson for her.
"The Christian life is risky," he said. "Ministry is risky. Taking on the devil is risky. What's riskier? Saying no to God. Say no to God and the Devil's gonna get you."
Larson said that sometimes the people who come to them to be exorcised are a little taken aback when they see how young the girls are.
"It's like, 'They're going to exorcise me?' It's just totally out of the box. But a few minutes into it, when they see the boldness and the confidence, the maturity and the knowledge of these girls, that all fades away," he said.
"We're not proud of ourselves," Tess said. "We're humble. We're still learning."
Nonetheless, there are very serious questions about the safety and morality of what the girls are doing for others, especially those who might need mental health care.
One woman, a grandmother who flew in from Dallas for an exorcism with the girls, told "Nightline" that she has demons who have physically hurt and raped her. She insisted she is not mentally ill, but admitted she had been on anti-depressants and had suicidal thoughts in the past. During the exorcism, the woman said her father sexually abused her as a child.
When asked if she thought the exorcisms could be making people with mental illness worse, Brynne disagreed.
"We do this under Dad's supervision. We never do it alone," she said. "He's been doing it for 30 years. He would know if something was going wrong."
However, Bob Larson has been accused of fraud and taking advantage of vulnerable people who are either desperate or prone to suggestion.
The Rev. Darrell Motal of the "Soul & Spirit" Para Church, who believes in the existence of demons, told "Nightline" that Larson is too quick to blame someone's problems on demonic possession and that it's more likely that Larson's clients need mental health care and spiritual guidance.
Father Edward Beck, a Roman Catholic priest, echoed Motal's comments and said he also believes that these young girls are "unqualified" and "unprepared" to perform exorcisms, and that it could be dangerous for them, as well as their clients.
While Larson admitted that he was not a mental health expert, he said if a demon is "blocking the therapeutic help, the therapy's not going to go anywhere significantly."
"Get the demon out, the impediment, and then the therapy can go forward," Larson said.
What's more, Larson and the girls' exorcism sessions are not free, and he insists that one session almost never does the trick.
"We have to fund what we do," he said.
Larson is currently weighing several offers for new reality shows starring Brynne, Tess and Savannah.