She only appears in the season 1 episode "Q&A" and she is voiced by Kath Soucie.
Yelena was a contestant of the show "Think Thank Thunk" and she was competing with Arthur Brown. Arthur lost the game and fell into a revenge driven depression while she became a world-class chess champion.
She was later captured by him, after her humiliation in public. Along with Ross Darren and Herbert Ziegler, she was later saved by Batman.
As the Batman pointed out, regardless who is the smartest between the Cluemaster and her, only the latter managed to accomplish admirable international recognition for her skills and talents.