A founding member of Boyzone told jurors his ex-girlfriend would go "crazy" over trivial problems, before she allegedly murdered her French nanny.
Former pop star Mark Walton, who is based in Los Angeles, said he never heard of 21-year-old au pair Sophie Lionnet before murder squad detectives contacted him.
Giving evidence at the Old Bailey, he said his ex, Sabrina Kouider, would "flip" during their two-year turbulent relationship.
He would support the fashion designer with thousands of pounds every month, even paying her rent long after she left him.
Mr Walton told jurors he "created" Boyzone in 1993 and was in the band for about a year before going on to be involved in Fifth Avenue.
By the time he met Kouider in 2011 he was doing well financially in the music business.
The Irishman told jurors they met in a NatWest bank in Notting Hill and hit it off straight away.
He said: "I was in love. She was my life then so..."
Mr Walton said he paid for Kouider's nannies but she would fire them over accusations of stealing and of being interested in him.
He said: "I actually challenged Sabrina on this. I did not believe her.
"I think some of them were friendly. I'm a friendly guy, but there was nothing more than that."
The musician said they lived together for two years in Queensway, London, before she disappeared.
He tried to rescue the relationship by paying for her to fly to LA, he added.
On their relationship, he said: "Sabrina shared some stories from her past.
"I guess knowing that, I felt it brought us closer together at times, but it was turbulent, probably the most turbulent relationship I had ever been in.
"She would go from softly spoken French accent then she would flip, get very angry, very loud and just not care where we were.
"She would just go crazy over something trivial."
In 2013, he paid £12,800 in rent on Kouider's Wimbledon flat but stopped the financial support in February 2014, the court heard.
The next time he heard of Kouider was when she accused him of being a "paedophile".
She rang his mother in Dublin, contacted his business partners and created a fake Facebook page, he said.
Referring to outlandish accusations levelled at him by Kouider, prosecutor Richard Horwell said: "Have you ever been party to a plot to drug the people in the Wimbledon flat and whilst unconscious sexually abuse the occupants?"
Mr Walton said: "Absolutely not."
He said the last time he was in the UK was when he went to a meeting in October 2015.
Jurors have heard Mr Walton lay at the heart of Kouider's campaign of torture against Miss Lionnet.
Kouider, 35, and her partner Ouissem Medouni, 40, allegedly beat her into a so-called confession that she was in league with Mr Walton.
Hours later, she was killed and her body thrown on to a bonfire in their garden in Wimbledon, south west London, the court has heard.
Mr Horwell asked Mr Walton: "We have heard that your name was referred to by Sabrina Kouider before and after her arrest. Had you ever heard of Sophie Lionnet?"
Mr Walton said: "Never, ever."
Mr Horwell asked: "Had you ever talked to her?"
He replied: "Never."
The lawyer said: "Had you ever communicated with her in any way?"
The witness said: "No way, never."
Kouider and Medouni have admitted perverting the course of justice but deny murder.
Cross examining for Medouni, Orlando Pownall QC said he would not be suggesting there was any truth in Kouider's allegations.
After Kouider would "fly in a rage at him", Mr Pownall said: "You must have asked yourself many times where did it go wrong."
He went on: "Would it be fair to say there was an occasion she accused you of not having any money? Did she run out into the street and shout?"
Mr Walton said: "Yes. In Oxford Circus. We were just shopping. It happened twice."
In 2012, police had been called a number of times over various accusations, including that he had photos of another woman on his phone, jurors heard.
Then in July 2014, she took out a non-molestation order, accusing Mr Walton of harassing her.