Sir Simon, or the ghost, is the namesake and major character in "The Canterville Ghost." He has roamed the interior of Canterville Chase since his death in 1584 and is a very complex and emotional character, for a number of reasons.
First of all, the ghost is fiercely proud of his personal history. He boasts, for example, of his success at the Kenilworth Tournament and the compliment paid to his suit of armour by Queen Elizabeth I. He loves to reminisce about his "brilliant and uninterrupted career of three hundred years" as the resident ghost of Canterville Chase. These recollections border on egotism, as the narrator states in Chapter 3, but are central to understanding the delight he feels at being able to scare others.
Secondly, the ghost has a theatrical nature which he expresses through his hauntings. He does this by creating characters, like the "Red Reuben" and the "Gaunt Gibeon," routines that he has developed to aid him in terrifying the residents of Canterville Chase.
His failure to terrify the Otis family, however, gives us a glimpse into a darker side of the ghost's character. In the beginning he becomes angry and determined to have his revenge. But, as his failures increase, his character softens and becomes more vulnerable. The opening of Chapter Four sums this up succinctly: "the ghost was very weak and tired...his nerves were completely shattered, and he started at the slightest noise." In other words, when the ghost is not fulfilling his purpose, he quickly becomes dejected and feels that he is not appreciated by those around him.
When he begs Virginia Otis to pray for him, so that he might have eternal peace, we see another important aspect of his character: the ability to be repentant and humble.
The ghost is, therefore, not at all how one might imagine a ghost to be. He is other-worldly, yet strangely human. He exudes pride and vanity, yet is vulnerable and feels a strong urge to repent of his past crimes.