Blue star Duncan James threatened a landlord with a crowbar after striking up a friendship with a woman accused of murdering her French nanny, a court heard.
Sabrina Kouider, 35, became "good friends" with the One Love singer after going out with Mark Walton, a founding member of Boyzone in 2012, the Old Bailey heard.
In September last year, after getting back with her financial analyst partner, she allegedly killed au pair Sophie Lionnet, 21, and disposed of the body on a bonfire at their home in Wimbledon, south west London.
The court has heard she became obsessed that Miss Lionnet was in league with Mr Walton against her.
Giving evidence, her partner and co-accused Ouissem Medouni, 40, told how he fell in love with Kouider at a French funfair, when she was aged 19.
Despite finding out she was seeing other men and concerns for her mental well-being, he went back to her and supported her financially, he said.
Medouni, who had worked for Societe Generale in London, said he had never been violent to a woman, but Kouider had been violent towards him "a couple of times".
The court heard police were called when she complained he was working too hard, making her feel "neglected", and on another occasion when she threatened to harm herself with a knife.
The defendant said he became "very concerned" about her mental welfare after she set light to a curtain.
In 2011, Medouni said Kouider told her she had met a man in the music business called Mark, but went on to claim over a coffee that he had attacked her.
He told jurors: "She told me she had arguments with Mark and he tried to strangle her and was very violent to her, so I was a bit shocked."
She then struck up a friendship with the boy band Blue, in particular Mr James, who she said was "nice" and a "good friend".
Orlando Pownall QC, defending, asked if he knew anything about the pop star's sexuality. Medouni said Kouider had told him Mr James "had both".
The barrister asked him about an incident when police were called to Kouider's north London flat to reports of damage to the door after she had left.
Mr Pownall said the landlord complained she and a husband or partner had told him to "f*** off" and "threatened him with a crowbar".
Medouni said that Kouider told him it was Mr James, who lived nearby.
As a couple, Medouni said Kouider was the "dominant" partner.
Her moods would go "up and down very quickly in the space of seconds", he said.
"In the last years, every morning she would wake up screaming for nothing, and, if she had a dream about me being with another woman, she said it was going to happen and make up a story in her mind about it.
"She said she had a gift for knowing what is going to happen," he added.
Medouni told jurors he did not kill Miss Lionnet by his own hand or make a plan with Kouider to cause her death.
Mr Pownall asked: "Did you cause her body to be burned in a fire in the garden at the back of Wimbledon Park Road?"
Medouni said "Yes."
The barrister said: "Do you agree that you made many dreadful decisions in the course of the account you are about to give?"
The defendant replied: "Yes."
Medouni said he covered up for Kouider's mental instability with police and social workers as she became more obsessed with Mr Walton.
He told jurors she was also "jealous" of him talking to Miss Lionnet, who he said was "very shy" and liked staying in her room reading Harry Potter books.
The defendant said Miss Lionnet had been paid £50 a week but the couple stopped paying her at the end of July last year because Kouider was "angry at her".
Medouni and Kouider have admitted perverting the course of justice but deny murder.