From Halopedia, the Halo wiki
Theresa was the product of Luther and Ramona's intense six-week long relationship in college. Ramona had kept her pregnancy with Theresa a secret from Mann, as she did not want to impose a burden on his academic life. Theresa's existence became known to Mann a year later when Ramona sent him a picture of their child. Luther Mann was never given the opportunity to be a part of Theresa's life, and his early attempts to find her proved futile. In 2555, Mann undertook a perilous mission to prevent the Halo array from activating. He was determined to succeed in order to ensure his daughter Theresa's survival. After successfully stopping Halo's activation, Mann concluded he had wasted time over the years and resolved that he had to go find his nine-year-old little girl.
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