Bassett will be playing Dr. Amanda Waller, who is a recurring character in a lot of DC books. She's not a superheroine, but a government agent who has headed up her share of superhero teams. She was the boss of the Suicide Squad, a team of former supervillains who took on dangerous missions in exchange for amnesty. She was also the the leader of Checkmate, DC's covert ops agency.
It's an interesting character to pop into Green Lantern, mainly because Waller hasn't played any part in Hal Jordan's storylines that I can remember. She's played more of a part in Superman and Batman stories (she even served under Lex Luthor in his unpleasant term as president) and has popped up in Justice League Unlimited and in Smallville. I imagine those who are fervently wishing for a Justice League movie might see this as hope Warner Bros and DC are going to start tying the universe together, despite what Christopher Nolan says. And who knows? Perhaps you'll be right in the end.