The actor, 51, has been charged with three counts of rape, three counts of sexual assault, and one count of indecent assault from 1999 to 2009 in what is expected to be a ten-week trial.
Brand denies all the charges.
Gallacher - who has three children with Brand - was this week twice spotted attending the trial at Southwark Crown Court in London alongside her husband.
The author and illustrator, who briefly dated Brand when she was just 19 before they reunited in 2015, is said to have made a “huge” impression in court with her unwavering loyalty.
Laura Gallacher has stood firm by Russell Brand’s side
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The 38-year-old is said to be “determined” to get through the legal battle by his side. The US-based couple have been staying in the Home Counties and spending time with her parents during the trial, The Mail reported.
“What Russell is being accused of was long before he and Laura got back together. It isn’t going to be easy, but they are determined to get through this,” a source said.
“Laura is absolutely besotted with Russell, she always has been. Being there for day one was a must for her.”
Brand was already hugely successful at the time, hosting his own Radio 2 show, presenting two Big Brother spin-offs and touring the UK with his comedy show, Russell Brand: Only Joking.
She is ‘besotted’ with him and ‘determined’ to get through the trial
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“Laura was besotted with Russell. She wanted to mend him,” a friend said.
“She had known him all of those years back when he was the man to date, but by the time she met him again he had been through the mill with Katy Perry.”
The pair crossed paths again eight years later in 2015 after both had suffered heartache.
“My ex had literally moved out of our flat that day and I was heartbroken, with mascara running down my face,” Laura told Grazia.
“A friend dragged me out on a walk to the canal in east London, where I’d never been in my life, and as we came on to the footpath Russell was standing there, like he’d organised it.”
She told the Made by Mammas podcast in 2020 that Brand seemed “so different,” and that they immediately knew that this time it would be serious.
“You have to rebuild your foundations and trust again, we did it the right way. It was very slow, it was getting to know each other and going out on day trips together.”
“I remember him saying: ‘I think I want a quiet life and a family,’” and it was exactly what I wanted. So, then it was about seeing if we could build a foundation to make that happen,” she told You.
Brand seen outside court on Wednesday
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The couple tied the knot in August 2017, marrying in a church ceremony near their home in Henley-on-Thames, Oxfordshire.
They are parents to Mabel, nine, Peggy, eight, and Herb, three.