Harrison is the best friend of Jillian Zinman. They have been best friends since she made him eat a bowl of mud in first grade. Although ever since that day Jillian fears that one day he'll try to get her back. Harrison joins Jillian when she's forced to take her sisters Katie and Amanda, as well as their doll Mary-Ellen to the Little Theater to watch a performance by Jimmy O'James and Slappy. After Katie and Amanda get insulted by Slappy, they run off backstage. While Jillian gives chase, Harrison hangs around backstage talking to some of the kids from their school who helped with production. He also found Slappy in the dumpster behind the theater. Thinking that Jimmy must have gotten rid of the dummy, Harrison takes him. He leaves Slappy with Jillian and her family.
Harrison and Jillian also try their hand at a clown act, performing at a five year old's birthday party as "Zippy and Zappy." However, all their gags fail to get a laugh, and when their whipped cream pie is replaced with stinging soap, it injures one of the kids at the party, causing the two to wind up not getting paid. Things get worse when the two kids begin to suspect that Slappy is really alive. They find Jimmy O'James' place, only to see the house has been abandoned. They do find Jimmy's diary that mentions Slappy's origins, as well as the spell to awaken him. More bad things befall Jillian, ultimately getting her grounded. But she can still host another party with Harrison. They try a ventriloquist act with another dummy named Maxie, but Slappy ends up replacing the dummy. Slappy is intent on marrying Jillian, much to the disapproval of Mary-Ellen, who was also alive and was the one sabotaging Jillian. The two dolls fight until they both destroy themselves with the table saw in Mr. Zinman's workshop.
After cleaning up, Harrison reads from Jimmy's diary that Slappy's spirit cannot be destroyed and that it can possess others. Jillian then suddenly vomits a green substance over Katie and Amanda, meaning she was likely possessed by Slappy. In the television adaptation, it's Harrison that winds up possessed by Slappy.
Harrison is described as being very big, but not chubby, with serious dark eyes and dark hair.
Harrison has a pretty laid back personality and is very easygoing, but doesn't seem to have the same enthusiasm about being an entertainer that Jillian has. He particularly has less interest in the clown act.