From Halopedia, the Halo wiki
Michael 'Sully' Sullivan was born on September 3, 2510 to Evelyn and Steven Sullivan. Sully's father Steven was a researcher for the United Nations Space Command, whose occupation often prevented him from seeing his family. As a result, Sully and his two younger brothers were predominately raised by their mother. Growing up without the presence of his father, Sully developed a laid-back demeanor and attitude. Feeling that his eldest son needed to learn proper discipline and structure, Steven Sullivan insisted that Sully attend a military school. Sully's mother disagreed with this mandate as it would mean sending him away from home. Nonetheless, Michael eventually enrolled at the Corbulo Academy of Military Science on Circinius IV in 2526, hoping to one day follow in his father's footsteps and also become a researcher.
When the planet was attacked by the Covenant in 2526, Sully was one of the only three survivors among the colony's residents. He later joined the UNSC Navy after graduating from another Officer Candidate School. During the Human-Covenant War, Michael gained the attention of the Office of Naval Intelligence and was assigned to Section One to put his skills in information gathering, knowledge synthesis, and cryptography skills to good use. In 2542, the UNSC abandoned Sullivan's homeworld of Emerald Cove due to the Covenant threat.
Following the Covenant War, Michael had risen to the rank of Commander and had moved to a leadership position in Section Two, taking on the public-facing role of Senior Communications Director. By 2558, Michael had taken up partial residence on Earth, maintaining an office in Boston, URNA.
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