First, Dickens makes the Ghost of Christmas Past part of an effective story by making him a vivid character. For one thing, the Ghost's appearance is quite distinctive and memorable. He wears a white tunic decorated with flowers, holds a sprig of holly, and a jet of light emanates from his head. Further, while he is the size of a child and unwrinkled, he has long white hair. Most curious of all, he fluctuates: sometimes he is whole and sometimes all Scrooge can see is an arm or a leg.
The Ghost also has a distinctive personality. He is authoritative and won't let Scrooge, who is unwilling to travel with him, off the hook. Second, he interacts with Scrooge in ways that encourage Scrooge to question his own assumptions. For example, when the Ghost sees how much Scrooge longs for Christmases past, he mocks him using Scrooge' own speech patterns:
A small matter,” said the Ghost, “to make these silly folks so full of gratitude.
Although that's exactly what Scrooge might have said a few hours before, now he realizes it is not such a small matter to be kind and generous to people.
The Ghost's interaction with Scrooge helps him to change into a better person, contributing to an effective story. Just like Scrooge, we are not likely to quickly forget this ghost.