Reptilla is the tyrannical ruler of Atlantis. She tried to conquer the world, until Atlantis was sunk by the original Kong, who was king of Atlantis.
In the present, Reptilla observed through a probe with her henchman, Lord Sycophis, that Kong had been cloned. She sent Sycophis and Minion to bring Kong to Atlantis, with the intent of making Kong her king to unite all of Atlantis and make them serve her.
When Sycophis caught Jason, Lua, Tann, and Zeelah sneaking into Reptilla's palace, Reptilla convinced Kong that they only thought of him as a pawn, and had Sycophis imprison them with the slaves, including animals captured from Kong Island. Reptilla controlled her slaves and many of her followers with mind bands, which she controls using the gem on her forehead, created through alchemy (a combination of science and the supernatural).
Reptilla planned to give Kong a crown containing a mind band so she would gain full control of him. During a rebel invasion, Kong realized Reptilla was evil, and went back to Jason's side. Reptilla sent giant reptilian monsters after Kong, but with Jason's help Kong defeated them. Reptilla tried to retreat, but collided with Minion, causing her mind control gem to fall off her forehead, releasing all those brainwashed, including Minion, who chased after Reptilla to take revenge on her.
Reptilla's fate beyond this is unclear, though as Kong sinks Atlantis once again, Zeelah seems to think she is still a threat, electing to stay behind to help the rebels defeat Reptilla once and for all. It could be that Minion either killed Reptilla and she didn't know about it, or Reptilla escaped and continued to command those loyal to her until she was inevitably defeated by the rebels now that she was outnumbered.