Bartimaeus of Uruk, or Sakhr al-Jinni of Al-Arish, is one of the main characters in the narrative, one of its two protagonists, and one of its two narrative voices. As a 5,010-year-old djinni, magicians deem Bartimaeus to be of the 14th level in their hierarchy of spirits. He once served notable figures such as King Solomon and Ptolemy I Soter, whose child form he often transforms into because he was fond of him. As a djinni, he doesn’t have a defined physical form, as he, like other spirits, is made only of his essence. When summoned, however, Bartimaeus can take a physical shape, but being in one for too long eventually causes him harm. His transformation powers allow him to take different forms, such as birds, underground creatures (like moles), gargoyles, and even people. As a spirit, he can also cast a number of spells, including Detonation spells and Concealment spells, at will.

While spirits do not display any notions of family, their outlook on friendships is heavily influenced by their routine enslavement to magicians. As Bartimaeus explains, “[W]e learned to avoid conflicts of principle. As a result, loyalties among us are temporary and liable to shift.