Clara Sesemann is a wealthy German girl residing in Frankfurt, daughter of the often-absent businessman Herr Sesemann. Her mother is deceased. Confined to a wheelchair by a chronic condition, she leads a sheltered, lonely existence under the strict oversight of her governess, Fräulein Rottenmeier, and household staff.

Heidi's arrival as her companion brings significant change. Despite different backgrounds, Clara forms an immediate, deep bond with Heidi. She delights in Heidi's vivid Alpine tales and free-spirited nature, finding joy and excitement that contrasts her own restricted life. Clara shows kindness and patience, frequently defending Heidi against Fräulein Rottenmeier's severity. She actively joins Heidi's lessons and cultivates a warm rapport with her visiting grandmother, Frau Sesemann, who encourages Heidi's literacy and faith.

Clara's transformation unfolds during her extended stay in the Alps. Immersed in the mountain setting, she thrives on fresh air, goat's milk, and natural beauty. Her health improves visibly; she sleeps soundly and gains strength. A pivotal moment occurs when jealous Peter pushes her empty wheelchair down the mountain, destroying it. This forces Clara to attempt walking. With Heidi's persistent encouragement, support from Heidi's grandfather (who carries her daily to the pasture), and her own gradual effort, Clara learns to walk unaided. Her triumphant first steps, witnessed by her overjoyed father and grandmother, mark a profound personal victory and a shift towards independence.

After recovery, Clara returns to Frankfurt but maintains her Alpine connection, pledging annual summer visits. Her experience fosters lasting gratitude towards Heidi and her grandfather. Herr Sesemann commits to providing for Heidi's future if required, cementing the families' enduring bond.