Answer:
B. Antagonist
Step-by-step explanation:
An antagonist is a character which has opposing ideas or traits as the protagonist. In the short story. Cammie is the protagonist around whom the story is revolving. Her action of planning to meet Greg stands is opposite to her brother. She wanted to meet Greg but her brother canceled their meeting by writing another email to him. This is the characteristic of an antagonist character. Hence, the character of Cammie's brother is the antagonist.