From Halopedia, the Halo wiki
In 2492 a series of incidents occurred at Luna OCS Academy on Luna. They involved the overnight disappearances of Inna Volkov from family officers' quarters on base, sightings of her in the company of a young man, and a child produced by those liaisons. On June 7, 2492 six cadets were brought before a Board of Inquiry to testify. One of these was a young Preston Cole. Though Cole was not found guilty of any wrongdoing, he married Inna two months later, with the admiral acting as a witness. They had a two-week honeymoon, followed by Cole being reassigned to the UNSC destroyer Las Vegas. The child, who was not Cole's biologically, was born December 12th.
In his report on Cole's life, Codename: SURGEON suggests three reasons why this marriage took place. The first was the admiral knew which cadet was the true father and didn't like what he saw, and thus found a suitable replacement for his daughter in Cole. A second was that the child was not the offspring of any of the cadets and thus the admiral's grandchild would have been fatherless. The third was that Cole had the liaison with Inna, though he was not the father of the child. Either way, Admiral Volkov either made Cole marry his daughter or else Cole was compelled by a sense of chivalry.
The two remained married and over the next eight years had two more sons and a daughter, which were Cole's biologically. However as the Insurrection flared up Cole saw very little of Inna and their children, being engaged on the frontier for months and even years at a time. Inna filed for divorce in 2500. Cole wrote to her, accepting responsibility for the failure of their marriage, stating that she never wanted a long-distance military marriage, and though he would always love her, he could not ignore his duty to humanity. He continued to write home to his children, but received no replies, leading him to believe that Inna was burning his letters.
List of appearances