Donny Cairn is the father of Lachlan White.
Scotsman Donny had a relationship with Chrissie White in the late nineties which resulted in Chrissie becoming pregnant with their son, Lachlan. At some point, Chrissie and Donny split and Donny didn't have much contact with Lachlan thereafter.
In April 2015, Chrissie contacted Donny after Lachlan sexually assaulted Alicia Metcalfe and was ordered to attend therapy. When successful businessman Donny turned up at Home Farm, he was shocked to learn what Lachlan had done and vowed to be there to support his son. Chrissie believed she and Donny were on the same page about how to deal with Lachlan, unaware that Donny was encouraging Lachlan to tell her what she wanted to hear to get her off their backs.
Less than a week after he arrived in Emmerdale, Donny was bundled into a back of a car and beaten up. When he returned to Home Farm after the attack, Donny lied he was beaten up due to what Lachlan did to Alicia, although Chrissie's husband Robert didn't believe it. It soon transpired Donny wasn't the successful businessman he portrayed himself to be and he was in thousands of pounds of debt to a man name Charlie. Donny planned to steal Chrissie's flash car and sell it off to pay his debts but a flat tyre scuppered his plan.
Unable to sell the car to pay off his debts, Donny played on Lachlan's fear of bullies and suggested Lachlan moved to Scotland to live with him as he knew Chrissie's father, Lawrence wouldn't allow Lachlan to go without. As Donny expected, Lawrence agreed to pay for the costs of Lachlan relocating to Scotland, which would allow him to clear all of his debts.
As Lachlan was preparing to move to Scotland, Robert's lover, Aaron Livesy, told Robert he overheard Donny on the phone to someone he owed money to. A few phone calls later, Robert discovered Donny was penniless and living in a bedsit after his business collapsed. Robert told Chrissie what he had found out and she threatened to tell Lawrence and Lachlan about Donny's plan to extort money from them. Donny stated he would deny it and twist things so Lachlan would believe she'd do anything to stop him from moving away and having a fresh start. Donny ordered Chrissie to get him £10,000 he needed to pay off his debts, or he would turn Lachlan against her and screw their son up further.
Chrissie began plotting against Donny and hired Ross Barton to help. Chrissie gave Donny the £10,000 he needed then Ross and Debbie Dingle followed Donny to Royal Grove Hotel where Donny was to hand over the money to Charlie. Whilst reluctant accomplice Debbie stalled Charlie and his thugs in the car park, Ross persuaded Donny that he was one of Charlie's thugs and got Donny to hand over the money.
As Charlie and his thugs walked into the hotel reception, Ross walked out with the money but he and Debbie couldn't get away as Debbie had lost the tow truck keys. Donny soon realised he'd been scammed and he and Charlie's thugs chased after Ross and Debbie. Ross and Debbie made a run for it and ended up hiding in one of the hotel's bedrooms. As Donny desperately tried to find out where Ross and Debbie were hiding, Chrissie phoned him and told him that he had been scammed. She told Donny she'd tell him where Ross and Debbie were with the money if he stayed out of Lachlan's life, otherwise she would put a hit out on his life. Donny called Lachlan and informed his son he could no longer live with him as he was moving to Thailand to look after his friend's bar. Chrissie then texted Donny a fake room number which allowed Ross and Debbie to escape with the money.
Whilst Lachlan was in prison for the murders of Chrissie and Lawrence, his best friend Gerry Roberts and Paul Tozer, he got back in contact with Donny using a secret phone. On Lachlan's instruction, Donny terrorized Lachlan's former girlfriend, Belle Dingle, who he blamed for ending up in prison. Donny ransacked the Dingle homestead, sent Belle anonymous texts and a birthday card and also killed the Dingles' pig, Gloria. As none of this has the desired effect, Lachlan ordered Donny to up the ante. Donny decided to set fire to The Woolpack with Belle and her family inside but he was caught in the act by Vanessa Woodfield. Vanessa tried to stop Donny and a struggled ensued. Donny grabbed a broken bottle and stabbed Vanessa in the abdomen then took off in her car, unaware Vanessa's three-year-old son Johnny was in the back. When Donny realised Johnny was in the car, he decided to leave Johnny at the side of a road. However, Vanessa's girlfriend, Charity Dingle, found them and rescued the toddler before having it out with Donny and beating him with a metal pole. When the police arrived, Donny denied any involvement in terrorizing Belle, stabbing Vanessa or setting fire to the pub and instead accused Charity of assaulting him. Belle realised Donny only terrorized her as Lachlan offered him money to do so. She told Donny told that Lachlan wouldn't pay up and urged him to do the right thing. Later, Donny visited Lachlan in prison and got his son to incriminate himself in the plot against the Dingles then revealed he was wearing a wire. Donny apologised to Lachlan then walked out.