Tom Yates was a somewhat major but yet minor character during the middle-later parts of the series. Before Frank Underwood’s rise to power, he was a popular author of some book called Scorpio among others. We later learn that much his legacy was fill with faux accomplishments and lies.
Frank hires him to do some kind of book based on his life. I never quite understood much of the logic in that project.
But he ends up leaving the Underwoods for some time until Claire convinces him to join the White House as a speechwriter.
And he becomes Claire’s romantic interest for the most part. He learns many of the Underwood’s secrets in their time together.
Which sealed his doom when Claire uses poison to end his existence after his loyalty proved to be untrustworthy. That was a good acting scene for Robin Wright, as I could tell how upset Claire Underwood was to do such a horrible act to someone she cared about. But she obviously cared more about her power.