While the two get along very well, their personalities are completely different.
Caster is far more energetic than her master and often times acts very silly and playful. Dispite this, she is very knowledgable about the world and knows when and when not to be serious. She is very affectionate towards Master and is not afraid to show it whenever and wherever (much to his embarressment). Her affection is not limited to her masters, but to others she's close with as well as things she finds cute. Caster is very perverted at times and often times ties to give Master hints through induendos. Other times, she will try to have her way with him by force and is again not afraid to do so in front of others. This is not limited to others if she finds them very cute or irrisistable (something that Master still hasn't quite gotten use to yet and a lot of times makes him jealous).
Unlike his servant, Master is a lot more calm and layed back. After his days of being a master for the Moon Cell Holy Grail War, he has a more simple outlook on life and has a "stop and smell the roses" additude. While he is very knowledgable on many subjects, his knowledge pails in comparision to that of Casters. He can be energetic at times, but has a bit of a hard time keeping up with Caster. Master often times thinks of Caster's behavior as exhausting and embarressing and is very quick to avoid or run away from Caster's affection. This often leaves others to believe that he doesn't share feels for his servant, which is not the case. Master is incredibly shy when it comes to Caster's affection and induendos, so he is often not sure how to react. This leads him to simply try to avoid the situation altogether to avoid embarressment. Even so, Master cares deeply for Caster and would do anything for her.