During the British Wizarding Civil War, Rosier worked as a double agent alongside his best friend Regulus Black; feeding the Death Eaters misinformation and giving the Order of the Phoenix inside knowledge of Voldemort's plans. However, unlike Regulus, Rosier was never a loyal death eater; being recruited by Albus Dumbledore himself to serve as a spy in his sixth year. In mid-1980, Rosier had been in danger of being discovered as a spy. As a means of getting Regulus closer with Voldemort's inner circle, Rosier agreed to have Black out him as a spy, and staged a duel before fleeing the country; sheltered by his friend Severus Snape. When Severus became involved in the war, Evan joined Severus' army and participated in the Battle of Hogwarts; helping evacuate the students from Hogwarts.
The war had a profound effect of Rosier, especially taking in account the loss of his best friend Regulus. Rosier came out of the war disowned and devoid of all material wealth. For the next twelve years, Rosier would live with Sirius Black and his family; taken in as a personal favour to his late brother. Never truly getting over Regulus' death, Evan was fairly depressed over the years, however he was finally persuaded by Sirius to be his assistant in his Defense Against the Dark Arts class in 1994. During his time as Sirius' assistant, he developed a close friendship with Gryffindor student Katie Bell, however their growing mutual romantic feelings for each other prompted Evan to resign from his position at Hogwarts in order to distance himself away from her.
Eventually, however, Evan relented and the two pursued a relationship, though not before Katie's seventeenth birthday. The two married not long after Katie's graduation from Hogwarts and had two sons; Lucas and Nicolas Rosier. Evan wrote a book on the British Wizarding Civil War about his first-hand experience as a spy for Lord Voldemort.