Roberta is ordered by Captain Moreland to cut her hair and dress up like a cabin boy named "Bertie" to avoid suspicions from Kuala and his crew. She is first seen lying on the beach of an island tied up with Captain Moreland after the pirates took their ship and captured them. She wishes they could fight and kill each other off when she sees that Captain Moreland is shocked to see them open a chest that belonged and was given to him by his late first officer.
When Kuala and some of his crew approach them, Captain Moreland does everything he can to talk his way out of this so as to avoid them trying to find out that "Bertie" is really a girl in disguise. Then, he makes a ransom deal with Kuala; a large amount of money in exchange for their freedom as long as Captain Moreland is the one who writes the ransom note. Kuala agrees to let them go after they have ransomed them and have been paid as promised. When he takes "Bertie" with them, Captain Moreland threatens not to write the ransom note unless they give her back to him. To make sure he gets what he wants, Kuala places her back with Captain Moreland and then returns to join the rest of his crew as Captain Moreland helps "Bertie" up.
Then Captain Moreland tells "Bertie" to keep quiet while the pirates decide which crew is going to take them. Then Fritz and Ernst come to their rescue. As happy as he is to know that help is on the way, Captain Moreland insists they free "Bertie" first. He knows that he is out of harm's way since he is too valuable for the pirates to ransom. As the pirates start fighting and killing each other, Kuala and some of them spot Fritz and Ernst helping Captain Moreland and "Bertie" to escape, so, he orders his crew to stop the fight and go after them. With "Bertie" free and Captain Moreland still tied up, the latter insists they leave without him no matter as the pirates get closer. So he stays behind as they hold him while the others go after Fritz, Ernst, and Roberta.
Roberta did not appear in the original novel. Instead, a young lady named Jenny comes to live with them towards the end of Isabelle de Montolieu's version. She was shipwrecked on a neighboring island. In Wyss' version, her name is Emily Montrose, an English girl found on Smoking Rock near the end. She is shy, but soon is adopted into the family.