How does his relationship get better with his master?
I believe you are referring to Pareja's relationship with Velazquez. Until Pareja is bought by Velazquez, he is treated cruelly and terribly abused. Slaves are considered to be property with no rights. Pareja's treatment before Velazquez demonstrates this. Once Velazquez buys Pareja, their relationship evolves over time into one of mutual respect and affection. Velazquez treats Pareja with sympathy and after a time, recognizes that he has been able to rise above the slavery into which he was born through his love of art and his desire to create art. This is why Velazquez gives Pareja his freedom in the end.