Lynn Russell, born Rin Midorikawa, is the half-English, half-Japanese daughter of George Russell, Viscount Marble, and Misuzu Midorikawa. Her appearance leans towards her English heritage, marked by wavy blonde hair. After her mother dies shielding her in a car accident, five-year-old Lynn moves from Japan to England, inheriting her mother's keepsake, the Lady's Key. She vows to become a "true Lady," embodying mercy, courage, and nobility.

Initially rejected by her aristocratic English relatives, including her paternal grandfather the Duke of Woburn, due to her mixed heritage, Lynn faces resentment even from her half-sister Sarah Frances Russell. She persistently seeks family acceptance. Financial hardship forces the Russells to lose Marble Mansion, prompting George to work tirelessly for its recovery. The sisters live apart—Sarah with the Duke at Woburn Castle, Lynn with Countess Isabel Montgomery at Montgomery Mansion—yet maintain their bond through shared golden heart-shaped lockets containing a family picture.

At eight, Lynn enrolls at elite Saint Patrick Academy, renowned for equestrianism, aiming to earn the coveted Lady Crest awarded to the top student. As a first-year equestrian club member, she forms a deep bond with Andrews, an unyielding Arabian horse previously mistreated by club captain Vivian Spencer. Lynn's compassion tames Andrews, earning her admiration but also rivalry and envy from Vivian and her allies, who see Lynn as a competitor for the Lady Crest. She befriends classmates like Zara and Dorothy, yet faces opposition, including blackmail attempts from Mary Waverly offering to restore Marble Mansion if Lynn abandons the award.

Lynn evolves from a cheerful, naive child into a resilient, principled individual, frequently demonstrating forgiveness. She reconciles with cousin Sophie Montgomery, who initially schemed against her out of familial pressure, after understanding Sophie's fear of abandonment. Similarly, Vivian transitions from rival to supporter witnessing Lynn's perseverance. Lynn also navigates a childhood romance with Edward Philip Brighton, her steadfast friend and a skilled equestrian. Her development culminates in winning a major equestrian competition and securing the Lady Crest. This victory fulfills her vow and reconciles her with the Duke of Woburn, who finally acknowledges her as his granddaughter.