In Spirit Bound, it's revealed that he never found her and had to stop since the guardians needed him more. At first, he tried and tried as much to find her, but knew how futile it was since the Strigoi disappear and do not want to be found. He first encountered Rosemarie Hathaway when he caught her taking a folder, in which she was trying to save someone. When she told him of her plan to save Dimitri Belikov and turning back to his original state, Mikhail couldn't believe how possible it was since he always read that Strigoi are undead and could never be cured, but when she told him that if she can find Sonya and turn her back into Moroi, Mikhail offered to help Rose in her mission.
In the Last Sacrifice, Mikhail plays a part in helping Rose escape from the court prison, and later in the book when Sonya Karp is saved from her Strigoi state, he is reunited with her. In The Golden Lily, Mikhail and Sonya are planning to get married sometime in the winter.
In countries other than America, Mikhail Tanner is called Tomas Sanders in Spirit Bound. This is due to a typo that wasn't noticed until weeks before production.