Sebastian and Charles share a deep friendship. While their bond appears to be platonic, Charles's descriptions often verge on the romantic. Their picnic in the first chapter of the book is portrayed in a dreamy and romantic manner:

We lay on our backs, Sebastian's eyes fixed on the leaves above, mine on his profile... and the sweet scent of the tobacco blended with the sweet summer aromas around us, while the fumes of the sweet, golden wine seemed to lift us just above the ground and keep us suspended.

Cara, Lord Marchmain's mistress, observes the closeness between the two friends and surprises Charles by addressing it in chapter four. She expresses her approval of relationships between young men, "as long as they don't last too long," and remarks that at their age, it's preferable "to have that kind of affection for another boy rather than for a girl."

The intensity of Charles and Sebastian's friendship has a profound impact on Charles. After meeting Sebastian, he changes his social circle at Oxford and even alters his room's decor and the books he reads to align with Sebastian's tastes. Charles also becomes closely involved with Sebastian's family, who come to see him almost as one of their own. Later, when Charles becomes romantically involved with Julia, Sebastian's sister, he reflects that Sebastian was the "forerunner," the first member of the Flyte family he loved. This strong connection makes Sebastian's struggle with alcoholism and depression especially painful for Charles.