Johnny "JJ" Johnson was one of Jim McDonald's army buddies from his days with the Royal Engineers. Johnny had a fling with Liz shortly after the twins were born in 1974, which was kept a secret from Jim for 22 years.
Johnny was a highly decorated solider, and served in the Falklands War. While serving, he was very close to Jim and family due to the McDonalds living in army housing. Once Steve and Andy were born, Johnny became their godfather and he later taught Steve how to ride a bike. Johnny also looked in on Liz whenever Jim was posted overseas. On one occasion, when Jim was in Norway, Liz was finding it hard to cope alone and found comfort in Johnny's arms. Their fling ended after three weeks and they agreed that Jim must never know.
In February 1990, the McDonalds were two months into a fresh-start at 11 Coronation Street, their first home in civvy street, when Johnny came to stay for a few days. Johnny was in Manchester for an interview for the position of recruiting sergeant at their office there - a job which, unbeknown to Johnny, Jim was also after. Out of respect to his mate, Jim didn't tell Johnny he wanted the job and afforded him every hospitality at No.11. Johnny was especially happy to see Liz again, but when she told him about Jim being interested in the position and made it clear that him living locally would be an unwanted complication for her, Johnny turned the job down. However, Liz still faced the unhappy prospect of Jim getting the job and remaining in the army, breaking his promise to her, and told Johnny that she'd finish with Jim if he took it. Jim considered this a bluff as a means of getting her own way, but Johnny warned him that she meant it this time. His friend's words hitting home, Jim decided not to pursue the job, and credited Johnny with saving his marriage.
Nearly six years later, Jim and Liz went to an army reunion in Macclesfield, where they ran into Johnny. Updating them on his life since leaving the army, Johnny told them that he'd spent some time in Australia and then introduced them to his wife, an Australian girl called Lucy. Liz found it awkward seeing Johnny again, and particularly when Johnny and Jim started drinking and certain facts threatened to slip out. Noticing a resemblance between Lucy and Liz, Jim made a comment to Johnny, who replied that he had a passion for redheads and started talking about Liz, calling her "the pin-up girl of the barracks". Johnny's words preyed on Jim's mind and, after incessant questioning, Liz admitted the truth about their fling to Jim on the way home. Heavily drunk, Jim threw Liz out of the car, called her a whore, hit her over the face, and drove off without her. This effectively ended their marriage as Liz refused to accept Jim's apology and started divorce proceedings a few weeks later.