Lynn Russell, born Rin Midorikawa in Japan, is a young Anglo-Japanese girl. Her mother is Japanese heiress Misuzu Midorikawa, and her father is English aristocrat George Russell, Viscount of Marble. Despite her mixed heritage, Lynn displays predominantly English features, including wavy blonde hair.

At age five, Lynn travels with her mother from Japan to England to reunite with George. A car accident during the journey claims Misuzu's life as she shields Lynn. George conceals this truth initially to protect Lynn. She later learns of her mother's death from Magdalene Waverley, a noblewoman scheming to marry George.

Following the accident, Lynn moves to Marble Mansion in England. She encounters her half-sister, Sarah Frances Russell, who initially treats her coldly due to lingering resentment. Undeterred, Lynn persistently seeks Sarah's acceptance. Their shared experience as motherless children eventually forges a strong bond. Lynn treasures her mother's keepsake, the "Lady's Key," vowing to honor Misuzu's wish for her to become a "true Lady" embodying mercy, courage, and nobility.

Life grows challenging when the Waverley family—Magdalene and her children Thomas and Mary—temporarily manage the household. Thomas and Mary frequently bully Lynn, while Magdalene openly resents her, blaming her for George's reluctance to marry. Lynn endures their mistreatment but finds support in Sarah, the household staff, and the neighboring Brighton brothers, Arthur and Edward. Under Arthur's guidance, she develops a passion for horseback riding, forming a special bond with his horse, Alexandra, whom she later rides skillfully.

Lynn's grandfather, the Duke of Woburn, rejects her due to her Japanese ancestry and pressures George to marry Magdalene to resolve the family's debts. Overhearing that George might agree to secure her acceptance, Lynn attempts to flee to Japan. Edward Brighton intervenes, riding Alexandra to stop her. George subsequently prioritizes his daughters over the marriage.

Three years later, unresolved debts force the Russell family to lose Marble Mansion. Lynn, now eight, lives with her great-aunt Isabel Montgomery, while Sarah resides with the Duke. Separated from her sister and father, Lynn enrolls at Saint Patrick Academy, a prestigious school emphasizing equestrianism. Her goal is to win the "Lady's Crest" award for outstanding students, hoping it will restore her family's honor.

At Saint Patrick, Lynn befriends the unruly horse Andrews, rehabilitating him after past abuse. Her growing talent attracts rivals: Vivian Spencer, a skilled equestrienne resentful of Lynn's bond with Andrews, and Mary Waverley, who returns seeking revenge for George's rejection of Magdalene. Vivian and Mary conspire to sabotage Lynn's Lady Crest candidacy. Mary cuts Vivian's horse reins during a race, causing Vivian severe injury. Lynn is falsely blamed and disqualified.

With help from friends and Vivian—who later supports her—Lynn gathers evidence to clear her name. Mary's grandfather, Victor Reynolds, brokers a deal: if Lynn wins an Olympic qualifier race, he will provide a loan to restore Marble Mansion. Lynn competes, riding Andrews, and wins before a crowd including the Duke, who finally acknowledges her as his granddaughter. She receives the Lady Crest and her mother's final gift, a tiara unlocked by the Lady's Key. The Russell family reunites at the reclaimed Marble Mansion.

Throughout her journey, Lynn remains resilient, kind-hearted, and devoted to her family. Her development centers on overcoming adversity, upholding her mother's ideals, and forging her identity across cultures.