Jackson "Jack" Bartlett, aka "Grandpa", is the glue that holds Heartland together. He's an ex rodeo cowboy, and cattle rancher. The last thing he expected was raising a teenager again during his golden years. He hates Tim because he hurt Amy's mother.
Jack was born and raised at Heartland, back when it was a working cattle ranch. He would help his grandfather around the ranch as much as he could. He promised to his grandfather to hold onto Heartland, all 600 acres. He spent years on the rodeo circuit, getting dozens of buckles and is considered a legend. He takes care of amy and Lou and is often jealous when Tim gets the girls presents.
Jack dies a slow death at the end of season 7, due to Tim carelessly playing around with a rake. Everyone leaves the ranch for Tim.