As noted, former WWE NXT Women's Champion Bayley was revealed to be Sasha Banks' mystery partner at WWE Battleground tonight. Bayley teamed with Banks to defeat Dana Brooke and WWE Women's Champion Charlotte.
While it appeared that it was Bayley's main roster call-up, JBL made it a point to say on commentary several times that his "sources" told him that Bayley's appearance was a one-time deal. He noted that Bayley was heading back to NXT to try and regain the NXT Women's Championship.
Below is a photo of Bayley and Sasha Banks after their match at WWE Battleground.