Martina von Trapp is the fourth daughter of Georg von Trapp and the late Agathe von Trapp, the sixth child among seven siblings: Rupert, Hedwig, Werner, Maria, Johanna, and Agathe. She is approximately four to five years old during the series' events.

Martina clings to her teddy bear, Nikola, carrying it constantly as a vital source of comfort and security. Episode 15 specifically highlights this profound attachment and the bear's anchoring role in her daily life, underscoring her youth and vulnerability within the family.

Her development includes moments of resilience and compassion. In Episode 18, she joins her brother Werner in secretly caring for a wounded fawn discovered in the forest, demonstrating empathy and nurturing instincts despite her young age. This subplot also results in her experiencing familial discipline, facing Denied Food as Punishment alongside Werner for sheltering the animal.

Martina's interactions are primarily seen within the family group, such as participating in musical activities and holiday traditions during the Episode 24 Christmas celebration. Her role becomes more central during the family's flight from Nazi-occupied Austria in the series finale (Episode 40), where she hides inside a coffin alongside sisters Hedwig, Johanna, and Agathe to evade German authorities, marking her most direct involvement in the escape plot.