How can histidine be classified both as positively charged and hydrophobic?
As @wysiwig stated, Histidine is not hydrophobic. Depending on the pH different parts of the molecule can carry charges, see the image below (from here):



The nitrogen atoms can be protonated in an acidic environment, the carboxylgroup will loose it proton in a basic environment. All these forms are transferred into each other, when the pH changes.