Friendship is one of the novel’s major themes. Harold immediately introduces Chester as his closest friend. He and Chester are united by being the only pets in the Monroe household. Although the two pets have very different personalities, they appreciate each other’s unique qualities and company. In the beginning of the novel, their friendship is built on routine. Each friend knows what to expect from the other. However, their friendship steadily unravels after Bunnicula’s arrival.

Bunnicula brings out an unexpected side of Chester. The cat is immediately suspicious of Bunnicula and fully believes that the creature is a vampire. Chester’s suspicions increase as he notes strange coincidences in the bunny’s appearance and habits. Harold has never seen this side of his friend before. Chester has always had a vivid imagination fueled by the books he reads, but this is the first time Harold has witnessed Chester’s imagination manifesting through paranoid actions. When Harold sees how Chester’s actions threaten Bunnicula’s life, he distances himself from his friend to save the bunny.

Harold’s relationship with Chester becomes strained, but he makes a new friend in Bunnicula. This newfound friendship causes Harold to experience a personal shift.