Cryer has had an on-screen presence on Supergirl since the fourth season, making him one of the series’ longest-running key bad guys, and he made an impression on the larger Arrowverse courtesy of the “Crisis on Infinite Earths” crossover that changed everything.
The prospect of Jon Cryer having a large presence in the back half of the final season indicates that he could very well be the ultimate big bad of the series, especially since Kara will be away from Earth for a while and not battling any baddies on the home front.