It was one of the central enigmas of The Blacklist Season 1: who is Tom Keen?
As usual, Raymond "Red" Reddington held the answers to that question from the start, but rather than outright giving Elizabeth Keen the answers he manipulated events so that they would unfold as she followed the threads down.
Thomas Vincent Keen (Ryan Eggold), formerly known as Jacob Phelps, was a child thief rescued from the streets 20 years prior by spymaster The Major (Lance Henriksen). The Major raised Jacob as a spy and infiltrator, working for various clients across the globe.
He was hired by Red (James Spader) to watch over Liz (Megan Boone), taking the name Thomas Keen and the cover of a schoolteacher. Red initially fired him from the job when he started developing feelings for Liz, so The Major switched his contract to Milos Kirchoff (Peter Stormare). He was allowed to remain with Liz as a result, though still as his target. They were married, and at the outset of Season 1 are planning to adopt a child together.
Tom: "I'm sorry. I can't. That's why I came here. To tell you, I can't do this, because I love my wife."
Jolene: "Wrong answer. Elizabeth Keen is not your wife, she's your target."
The Season 2 finale completely changed the landscape of the show; when Season 3 rolls around October 1st Red and Liz will be on the run from the FBI together as they search for a way to clear her name.
Initially it looked as though Liz and Tom were going to reconcile towards the end of Season 2, after he returned with the intention of sacrificing his freedom for hers. Instead of running away with him as planned, Liz left again to fight the Cabal with Red and Tom sailed away into the sunset on his boat.
However; this may not be the end for the star-crossed lovers as showrunner Jon Bokenkamp teased in the run up to The Blacklist Season 3.
"They're completely flipped in who they are and almost also in the way they approach the situation — she's now feeling the desperation of being a criminal and trying to operate in the shadows, and he's trying to come clean and trying to redeem himself in her eyes. Tom was sailing off to white sands and blue beaches and felt it was over. What he doesn't know is she assassinated the attorney general of the United States. When he catches wind of this, he'll have something to say about it and will not want to sit back."
Tom returned to help Liz once before; once he finds out what she's done and that she's on the run, will he give up his newfound freedom again to return to her? Meghan Boone (who plays Liz) seems to think so, as she's previously stated that she believes Liz is still in love with him.
Despite Tom's previous assertions that being married to Liz was just part of his job he's shown that he does care about her, professing to be one of the "good guys". Particularly with Bokenkamp's comment that he's going to be trying to "redeem himself" in Liz's eyes, it seems very likely that he'll be making a return to the show sooner rather than later.
This would certainly chime with the new dynamic promised for Season 3, though I can't imagine Red would be too happy to have Tom tagging along with them on their journey, especially if all those "Lizzington" shippers are to be believed.