Alistair 'Ali' Marsden is the youngest son of Ronnie and Frances Marsden, and twin brother of Elaine Marsden. The family moved into the house formerly owned by the Reynolds family. Ali immediately took a liking to Katie Sugden, angering her boyfriend Andy Sugden.
Ali tried to get Ollie Reynolds and Donna Windsor sympathy by telling them he had a heart defect, which meant he was dying. Ali also faked his heart defect kicking in, to get more attention. Ali showed how cruel he could be by spreading nasty rumours that Stephen Butler was gay, which led to Stephen's departure from the village.