Camden Pratt QC, for the prosecution, suggested that the relationship might not be easily explicable. "But the roots of this crime lie in that complex relationship."
Mr Jenkins, 40, a deputy headmaster, denies murdering Billie-Jo as she was painting the patio windows of the family house in Hastings, East Sussex, last year.
Making his closing comments on the 17th day of the trial, Mr Pratt dismissed the likelihood of the killing being the work of a stranger.
"To suggest a motiveless passer-by arriving and suddenly deciding to commit a crime doesn't allow for common sense to prevail in this case," he said.
Mr Pratt told the eight men and four women jurors that they had to focus on the time between 3.13pm and 3.25pm of Saturday, 15 February, when Billie-Jo was murdered.
The prosecution has alleged that Mr Jenkins bludgeoned the teenager with an 18-inch spike cleared out of the utility room earlier in the day. It was alleged that he then went on a pointless shopping trip with his two elder natural daughters, during which he bought nothing, to distance himself from the crime.
Mr Pratt went on: "Or did an unnamed, unknown, unseen person, a stranger, for no apparent reason, approach the patio? Did they decide to see what weapon might be about and, without any reason, attack Billie-Jo on the patio, and having murdered her, leave without a trace at all of ever having been there?"
Billie-Jo was a girl of nearly 14 with a disturbed childhood which had led to her being fostered. She was described by Mr Jenkins as being impatient and she had problems, Mr Pratt said. "She clearly sought attention from the defendant."
But, he said: "We are not talking about a gangster with lots of enemies. We are not talking about an individual with a shady private life. We are talking about a young adolescent girl."
Mr Pratt raised inconsistencies in Mr Jenkins's account of what he did after the shopping trip when Billie-Jo's body was discovered by his natural daughter, Lottie, then 10.
Mr Jenkins has admitted to Lewes Crown Court that he lied to emergency services staff about putting Billie-Jo in the recovery position and checking for her pulse.
He spent much of the time before the ambulance arrived with Lottie and her eldest sister, Annie, then 12, outside the room where his foster daughter's body lay.
"Why was [the recovery position] not done? So shocked he couldn't help, or not done because he knew that she was dead? Not done because his interest was not in keeping her alive? There is a limit to how far shock explains his conduct."
Mr Jenkins also denied seeing the blood-covered spike which lay next to the body.
Mr Pratt asked: "Are we dealing with a man who is now either deliberately or through some mechanism of denial not prepared not only to face the sight of the body, but not even the sight of the weapon that the Crown say he must have used to kill her?"
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