Dimitri is a new character. He is the son of Megan Hathaway, Stefano's daughter, who was "killed" back in 1984, but resurrected for Beyond Salem and then brought back to Days.
Dimitri is gay, but had to get married before his birthday to get money from his father's side. He managed to schmooze Gwen and she married him a week after they met. Meanwhile, Dimitri is secretly having sex with Gwen's bestie, Leo Stark.