Francis Dereham was a member of Agnes Tilney’s staff. Tilney was the Dowager Duchess of Norfolk, and step-grandmother to Kathryn Howard. Since the young Howard girl was living with her step-grandmother, the two had the chance to meet. However, Dereham was definitely not her first interest. Kathryn had already shown an interest in her music teacher, Henry Mannox.
It was her relationships with Mannox and Dereham that caused the initial search into her background. Henry VIII believed that his fifth wife was a virgin when he married her, but it turned out that at least one other man had carnal knowledge of her—Francis Dereham. Doing the right thing, Dereham admitted to the treason, but he made it clear that there was a promise of them being husband and wife. Dereham had left for Ireland for business, but he promised Kathryn that he would marry her upon his return. They had already called each other husband and wife.
When Dereham did return, Kathryn Howard was already married to the King of England. He was not going to tell the King about the pre-contract! Unfortunately for him, Kathryn denied the pre-contract but did admit to calling him her husband and lying with him. Had she admitted to the pre-contract, all their lives may have been saved.
Rumours spread that she wanted to rekindle her romance with the man, and that was her reason for making him her personal secretary. There is no proof of this, and it is possible that she simply wanted to care for her friends. The Tudors shows Dereham blackmailing the Queen, but this is unlikely. While she would have had a long way to fall, he would have also fallen by sharing the news of their previous relationship with the King of England.