Jack Hopper was Jackie's grandfather.
Jack suffers from Alzheimer's disease so is no longer able to care for Jackie and cater for her needs, and Jackie is not old enough to help her grandad as he is getting old. Because of this, Jackie is brought to the care home and her grandpa Jack is sent to a nursing home. Jackie kids herself that her beloved Grandad has nothing wrong with him and does not need to be kept in a nursing home, even though the effects of Alzheimer's disease are quite clearly shown in Jack's behavior.
When Jack was young, he was a well known athlete and Olympic medalist, specializing in long distance running. Jackie looks up to her grandpa for this reason and aspires to be just like him. Grandpa Jack means a great deal to his granddaughter, this is because Jack is all the family that Jackie has left. The reason for this is unknown. Jack gave Jackie one of his medals, which she carries around in a old, rusty tin, which Michael (Jackie's arch enemy) has his eye on but gets a shock when it is filled with worms and left on the bed as a trap from Tracy, Crash and Jackie to Michael to pay him back for being a nuisance.
Once Jackie is settled in to the Dumping ground and befriends Crash (whom she describes as 'the only one that makes any sense' in the care home) and Tracy Beaker, she visits her Grandad in his care home and he seems confused and at one point asks crash 'who's your friend?', directed at Jackie. Jackie still refuses to accept that her Grandad is ill.
Grandad Jack features in many episodes, including one where he and Jackie decide to take part in a running race. Unfortunately, Jack is unable to complete the race as he collapses on the track and is covered in an emergency blanket. However, he encourages Jackie to leave him and complete the race, which she proceeds to do. He also features in an episode where there is a party at the dumping ground and Jack comes along in his wheel chair , but he does not enjoy himself until right at the end where crash throws his slippers in the bin, puts his running shoes on for him and runs while pushing his wheelchair, while Jack times them with a stopwatch, to get him into an athletic, happy mood.
Sadly, in another series, Jack falls very ill and dies, leaving Jackie devastated. The other children at the care home are determine to help Jackie have a pleasant funeral for her grandpa so track down the friends that Jack used to train with. With help from Lol's 'Skills', Crash and Tracy manage to track down three of Jack's athlete friends, including Flying Freddie Mercer and Arthur McDonald, who attend Jack's funeral in their Olympic running gear.