The beloved friend of our six narrators, Percival never gets a chance to speak for himself, but his presence looms super large throughout the novel. If his friends are to be believed, the sun rises and sets around Percy: he is athletic, academically gifted, and pretty much admired by everyone.
As a result of these qualities, he's a natural leader. The other boys are so crazy about Percival that they try (and fail, according to Neville) to mimic even his tiniest gestures, such as the way he flicks his hand to the back of his head. When Percival enters a room, the narrators fall into order like obedient troops. For example, when all seven protagonists meet up immediately prior to Percival's departure, Bernard notes:
We who yelped like jackals biting at each other's heels now assume the sober and confident air of soldiers in the presence of their captain. (4b.18)
But Percival is thrown from a horse and dies while working in India, sending the lives of the six narrators into depressing tailspins. In addition to their sense of personal loss, his friends mourn the exceptional man they believe Percival would have been. As Bernard put it,
He would have done justice. He would have protected. About the age of forty he would have shocked the authorities. No lullaby has ever occurred to me capable of singing him to rest. (9b.7)
Percival is the novel's purest foil. Although the narrators compare themselves to pretty much everyone else—not just Percival—Percival is presented as most diametrically opposite to the others. As Bernard notes right after Percival dies,
Now, through my own infirmity I recover what he was to me: my opposite. […] My own infirmities oppress me. There is no longer him to oppose them. (5b.13)
Percival's death is an important focal point in the book's Jupiter-sized obsession with mortality. Though this preoccupation is present from the very beginning of the book, Percival's premature death forces the characters to confront the reality of death—and the abrupt arresting of a life's story that it entails—head on.
It should be noted that this character was incredibly personal to Woolf, as he is based on Thoby Stephen, Woolf's brother. Thoby also died unexpectedly, and great things were expected from him.