In season four, Aidan Macallan, played by Damian Young, is a brilliant, dance-loving data analyst and friend of LeAnn Harvey (Neve Campbell) who gets caught up in the plot to help the Underwoods win the election. With his help, the Underwoods are able to look at illegally obtained voter data and figure out who they should target so they can sway the campaign in their favor.
As season five goes on, Aidan gets deeper and deeper into criminal activity on behalf of the Underwoods, which leads to the FBI, CIA, and NSA taking an interest in him. With threats closing in on all sides (including a pretty serious one from Russian President Viktor Petrov), he decides to make a run for it, and LeAnn gives him a gun for his own protection the night he's supposed to leave on a flight out of town. Sadly, he's found dead in his hotel room later on, shot in the head with the very gun LeAnn gave him.
We can't be exactly sure how his death goes down, but it's heavily implied that Jane Davis, the deputy undersecretary of commerce for international trade, has something to do with it. In fact, she even lets LeAnn know that she has her gun to return to her.
