It was love at first sight for a 25-year-old dragon furry who fell for a teen who called herself Day Dreamer Fox Wolf.
But after her parents forbade the two from seeing each other because she was underage, he concocted a twisted plan to wipe them out, a California court has heard.
Frank Felix, 33, who went by the fursona Entey, met then 17-year-old Katlynn Goodwill Yost at a 'furry convention' – an event for people who get their kicks from dressing in elaborate animal costumes.
Felix knew Katlynn and her parents through the furry community, where her mother Jennifer acted as a shoulder to lean on for other furries to help them cope with their problems.
Felix's accomplice, Josh Acosta, 29, also had a connection to the family and was a known 'Brony' – a man who's a fan of My Little Pony. DailyMail.com has learned that Felix's fursona was a dragon.
Prosecutors say Frank Felix, 33, was the mastermind behind the 2016 triple homicide. He was escorted into Orange County Superior Court for opening statements on Wednesday
Katlynn Yost, 17, waited in the car with Felix as her mother Jennifer and step-father Christopher were killed
Felix (left) and Acosta began plotting their deaths over email after Jennifer told the then 25-year-old Felix he couldn't see her daughter anymore
The 'furry community' creates 'fursonas' that resemble animals with human qualities
Felix is charged with three counts of murder and faces life in prison without the chance of parole if found guilty.
Acosta was found guilty on all three counts of murder on November 8, 2018 and sentenced to life in prison on December 14 that same year.
After discovering her daughter's relationship with Felix, who was then 25, Jennifer, 39, told her she must stop seeing him, the court in Santa Ana, California, heard.
Her friend Melinda Giles, who's also a furry, told DailyMail.com back in 2016: 'To protect her daughter, Chris and Jenn were like, "We don't want you to be friends with our daughter anymore".'
After their relationship ended, Felix and Acosta, who was then 21 years old and a private first class in the Army stationed in Fort Irwin, plotted to free Katlynn from her parents, the Daily Bulletin reported.
The obsession, after their relationship fell apart, grew to the point of self-mutilation. Prosecutors described how in one instance, Felix cut himself in front of the teenager.
During the week leading up to the murder, he allegedly planned their deaths with Acosta through email.
They carried out this plan during the early morning of September 24, 2016.
The bodies of Jennifer, her husband Christopher Yost, 34, and their family friend Arthur 'Billy' Boucher, 28, were found by their then 6-year-old daughter in their Fullerton home.
Their 9-year-old daughter was also in the house at the time of the murder, but was unharmed.
They carried out their plan during the early morning of September 24, 2016 using Felix's dad's shotgun – cutting the security lock with bolt cutters they had purchased that week
The community is mostly online, but gather to meet each other at conventions such as the Anthro Weekend Utah furry convention
Katlynn and her mom Jennifer were both in the furry community. The 17-year-old's fursona was named Day Dreamer Fox Wolf
Felix drew his own dragon 'fursona' Entey on deviantart
The prosecution laid out a week-long timeline that, they say, showed the murder was premeditated, during opening statements at the Orange County Justice Center.
They purchased bolt cutters, ear plugs and shotgun ammo in Burbank, California.
The prosecution stated Felix wrote to an acquaintance: 'I might be assisting someone in murder.' When advised by the friend not to go through with it, Felix switched up and said he was talking about an animal.
They also noted the defendant was 'nervous' in the email exchanges with Acosta where they detailed their plans. In one instance, they wrote whoever answered the front door would either be pushed aside or shot.
Acosta was the one who agreed to carry out the murders after Katlynn told them she had been molested by her stepfather Christopher on a weekly basis – she would also testify about this in the former army mechanic's trial in 2018 in exchange for immunity.
The 17-year-old let them in the house and waited in the car with Felix as Acosta entered the residence.
Katlynn's mother Jennifer was a pillar in the community and often let other furries lean on her during their troubling times
Acosta agreed to carry out the murders. He shot family friend Arthur 'Billy' Boucher, 28, on the couch and Jennifer in the head as she was sleeping
Acosta gunned down Katlynn's stepfather Christopher after he tried to make a break for it out the back door
Using Felix's father's shotgun, Acosta shot Boucher in the head as he slept on the couch, then entered Jennifer's bedroom where he shot her between the eyes as she slept and finally gunned down Christopher as he tried to make a break for it out the back door, showed photos obtained by DailyMail.com.
After the couple's young children found their bodies, the teenager was regarded as missing until she was found later. Authorities at the base where Acosta was stationed found the three shotgun shells that were later tied to the killing.
Felix told detectives that he 'knew she wouldn't be safe if the parents weren't killed,' the Daily Bulletin reported.
Officials on the Army base in Fort Irwin where then 21-year-old Acosta was stationed found the three shells that were later connected to the murder
There are said to be some 2.5 million furries worldwide
Felix's defense attorney David Paulson declined to make an opening statement on Wednesday, electing to speak later in the trial
The furry enthusiast also admitted he knew the kids were home and planned to leave food out for them after slaughtering their parents, the prosecution said. They also knew that Boucher would be staying the night, with Felix admitting he knew there'd be a third body by the end of the bloodshed.
Acosta's defense during his 2018 trial argued the pair had been manipulated by Katlynn into carrying out the murders. It's unclear what angle Felix's defense will take as they've declined to speak until further into the trial.