Myrta is just Holmes' type: young, with blue eyes and a lush figure. Holmes had met her years prior in Minneapolis and writes to her, asking her to move to Chicago and marry him. To Myrta, the handsome young doctor probably seemed to have stepped from a world far more exciting than her own.
She accepts his offer. Bad move, Myrta.
The two marry in 1887 and poor Myrta has no idea that Holmes is legally married to Clara Lovering, the original Mrs. Herman Webster Mudgett. Myrta helps run the drugstore. But she quickly becomes uncomfortable by the endless train of young women piling into the store. Pregnant, she becomes possessive of Holmes, which makes him only see her as an obstacle. He makes her stay upstairs to manage the store's books so she's out of the way.
Holmes remains married to Myrta, though he keeps her and their daughter Lucy out of the way. The two remain legally married, though we never really hear much about her after Holmes' hotel basically becomes his life.