In Family Court, they are called "paramours," and they can play a major role in child custody cases.
Live-in boyfriends are eyed suspiciously, children's advocates and attorneys say because they are often the most dangerous person a child encounters.
Judges frown on unmarried couples living together so much that some have been known to invoke the state's 200-year-old fornication law to encourage a single mother to avoid allowing a boyfriend to stay at her house.
After Justin Hillerby's arrest Wednesday in the death of his girlfriend's 22-month-old son in Summerville, local experts familiar with the court system say the situation is all too familiar.
"It is a recurring problem," says Stephen Dey, a local attorney and co-chairman of the Family
Court Liaison Committee of the Charleston Bar Association. "In a vast majority of cases, if a person is going through a divorce, there is a proscription to exposing children to the situation."
Dey says he will not take a case involving a parent with a live-in lover because "your chances of getting relief from the court is slim to none."
That's because of the troubles they've seen in Family Court. National studies indicate children living in households with unrelated adults are nearly 50 times as likely to die of inflicted injuries as children living with two biological parents.
Laura Hudson, executive director of the South Carolina Crime Victim's Council and a member of the state's Child Fatality Review Team, said Thursday it's hard to break down state statistics to illustrate the problem, but "I'll say it's not uncommon."
Often, she says, the death of a child comes at the hands of an adult caregiver who is not related to the victim. What police say occurred in the Hillerby case is nearly identical to a 2007 St. George case. Donald Joseph Mauldin last year was sentenced to 50 years for killing his live-in girlfriend's 3-year-old daughter.
The Dee Norton Lowcountry Children's Center has seen a 28 percent increase in child-abuse cases in the last four years. Dr. Libby Ralston, the center's executive director, says they have a lot of experience with cases involving live-in boyfriends.
"A lot of live-in boyfriends don't have the affection or the bond with a child, and no experience as a parent," Ralston says. "We have found that the risk to children increases when they are exposed to multiple adults."
And it is even worse when that is a person who doesn't understand a child's behavior, or how fragile they are.
"They may even feel jealous of the relationship the child has with its mother," Ralston said.
Some states have even considered harsh penalties against parents who move in with a lover. In Iowa, the state legislature has considered a law that could cost a parent custody of her children if she allows a boyfriend to move in. Iowa lawmakers say 26 percent of child deaths in that state between 1995 and 2000 involved live-in lovers.