Supergirl is going in a new direction with its supervillain in Season 4. New bad guy Agent Liberty will be a human rather than an alien with all manner of superpowers, and his stance on the alien presence on Earth will make him an enemy of Supergirl and Co. The show doesn't intend to just introduce Agent Liberty -- a.k.a. Ben Lockwood -- as a baddie with no backstory. Instead, Supergirl has cast a Walking Dead star to play Agent Liberty's father. Xander Berkeley is on board Supergirl.
Xander Berkeley has spent the last few years playing the traitorous Gregory on The Walking Dead, and his character is still among the living as of the end of Season 8, so Berkeley may be pulling double duty between the zombie apocalypse and the Arrow-verse for a bit. On Supergirl, he's playing a character by the name of Peter Lockwood, and it's not difficult to understand how Peter inspired his son's malicious ways.
Peter Lockwood is a factory worker firmly in the middle class, but he's having a hard time making ends meet due to changes in the way the world works on Earth-38. Culture and society have shifted because of aliens living on Earth, and it sounds like he's going to be bitter that his fortunes have fallen due to a perceived injustice courtesy of aliens on the planet. He spent his life working to provide for his family, becoming proud of his ability to work and give his loved ones what they need.
While his single-mindedness likely served him well earlier in his life, the focus on his dislike of aliens on Earth will almost certainly yield unfortunate results. TV Insider reports that the "seeds of his discontent sows the beginning of an anti-alien wave," and the footage for Season 4 released so far makes it clear that his son is riding that anti-alien wave.
There's no news at this point of how much of Peter Lockwood we'll see in Supergirl Season 4, so it's entirely possible that Xander Berkeley will only show up for a guest spot. Given that Sam Witwer will be a series regular as Agent Liberty, however, I wouldn't be shocked if Berkeley pops up more than once. If Agent Liberty is as nuanced as Reign or any of the past villains, more than one episode of backstory could only be a good thing to flesh him out. After all, Supergirl has never delivered a supervillain like Agent Liberty.
Only time will tell. We can't say just now when Xander Berkeley will appear on Supergirl, but he'll probably pop up on The Walking Dead when that show returns to the fall lineup in the not-too-distant future. Stay tuned to CinemaBlend for the latest in TV news and developments on new characters coming to the Arrow-verse.