Daisy Lewis is one of Downton Abbey's new faces and will help Tom Branson - played by Allen Leech - to get over the loss of his wife Lady Sybil.
The actress has been brought into the show as the live-in-governess of Tom's daughter - though her character name isn't yet known.
Former chauffer Tom will spend the series raising his young daughter Sybil, who was named in tribute to his dead wife.
The 28-year-old newcomer's character will help mend the heart of the lonely man.
A source told The Mirror: “Daisy will become one of Downton’s most prominent characters. She wowed producers in her audition and they are delighted to have secured such a brilliant young actress.
“Her romance with Tom will be one of the main storylines of the series. He will begin to develop feelings for her and they start a secret affair.
"Tom will have an even bigger role in the new series after becoming the estate manager of Downton.
"The writers know he is a very popular character and the viewers were gripped by Lady Sybil’s traumatic death.
“One of the new maids tried to kiss Tom although, at the time, he wasn’t quite ready to move on.
“But viewers will see that’s all going to change in the new series.”
Daisy has appeared in Doctor Who and in the 2011 British film Pusher.
She first met Downton creator Julian Fellowes when she appeared in the 2009 film From Time To Time, which he directed.
Daisy is one of five new cast members joining the period drama.
Big names include Oscar-nominated actor Paul Giamatti as the playboy brother of Lady Cora, and opera singer Dame Kiri Te Kanawa who will play a house guest.