Batman Beyond: Sable Thorpe

Sable Thorpe was Paxton Powers's personal assistant, who betrayed him in league with her secret lover, King. 

Batman Beyond: Aquagirl

Aquagirl is the future daughter of the present day Aquaman. She has inherited all her father's powers, including telepathic communication with sea life and manipulation of water. When Batman is asked to join the Justice League, she is initially the only member who is friendly and accepting towards him.

Fun fact: Aquagirl is voiced by Jodi Benson, the voice of Ariel from Disney's The Little Mermaid.
Two more DCAU posts on the way, so stay tuned!