Crystal is a young girl in her midteen years. She tends to follow around Dave as she has a form of care and compassion towards him. Even though Dave asks her to stay away from him because he doesn't want her life to be in danger she persists to follow after Nero. Meaning she is kind of Nero's only companion throughout his journey even though Nero opposes the idea of a crew most of the time. He puts his ideology aside and allows her to be apart of his "crew." She doesn't have a Devil Fruit ability but has access to Haki. Meaning even though she might not be the strongest lady she can still put up a fight against anyone who tries to attack her. Whether it be her using Observation Haki to know where her enemies are or using Armament's Haki to give herself an armor around her weapons and bodies to seriously damage her enemies. There might be a few reasons why Nero keeps her around, but we all know for sure that she is definitely a good aid to Nero.