Chronos
Chronos is a time-controlling villain and an enemy of Atom (Ray Palmer). When Ray Palmer retired from being Atom, Chronos wasn't seen for awhile. Some years later as seen in "Sword of the Atom," Chronos resurfaces causing Ryan Choi and Aquaman to look for Ray Palmer. By the end of the episode, Ryan Choi returned to being Atom and helped Batman and Aquaman defeat Chronos.
Read more about this topic: List Of Batman: The Brave And The Bold Characters, Villains