Jeffrey Claremont, the second son of the Ninth Earl Claremont, grew up alongside his cousin Richard Ranasinghe de Vulpian like a brother. He possesses blond-brown hair, blue eyes, and a tall stature, bearing some resemblance to Richard. His personality blends charisma and cunning with a strong workaholic tendency and deep protectiveness toward family, often at his own expense.

Born into British nobility, Jeffrey disappointed his parents with a rebellious punk phase in his youth. His profound closeness with Richard defined much of their lives, making Jeffrey the most significant person in Richard's world. This bond fractured when Jeffrey, citing familial pressures and inheritance complications, encouraged Richard's fiancée Deborah to end their engagement. Jeffrey carried lasting guilt from this act; though Richard forgave him, their relationship never fully healed, often marked by tension and sharp words.

Jeffrey's devotion extended to his older brother Henry, the Tenth Earl. He habitually dimmed his own capabilities to ensure Henry received more attention and recognition, a dynamic that inadvertently fueled Henry's insecurities and created distance. Later, Henry discovered Jeffrey's secret romantic relationship with Joachim Bergmen through emails. Henry responded supportively, dismissing a blackmailing servant and urging Jeffrey to introduce Joachim to the family, gradually mending their fraternal bond.

Jeffrey and Joachim Bergmen began with a mutually beneficial, rivalry-driven association that evolved into friends-with-benefits. They promised not to "make the other happy," rooted in shared guilt and self-loathing. Over time, they confronted their feelings and committed to a relationship. Joachim often accompanied Jeffrey on global travels and selected his outfits due to a keen interest in fashion. Their sly, trickster-like natures earned their dynamic the description of "a kitsune and a tanuki dating."

Within the Claremont inheritance conflict, Jeffrey played a central role. The dispute centered on a gem purported to be a diamond in their great-grandfather's will, later revealed as a less valuable white sapphire. This deception aimed to shield the great-grandfather's son (Jeffrey's grandfather) from familial racism after marrying Leah, a Sri Lankan woman. The will's conditions—requiring marriages approved by family trustees—directly impacted Richard's engagement and fueled the family rift. Jeffrey later facilitated Henry's adoption of young relative Octavia Manorland by arranging their mother's formal adoption, ensuring Octavia's future as the Claremont heir.

Across the series, Jeffrey evolves from a figure mired in guilt and self-sacrifice toward greater emotional openness, particularly through his relationship with Joachim and reconciliation with Henry. Despite residual tensions with Richard, he remains integrated within the family, participating in key events like Henry's ascension as Earl.