Friend of Emma. Charlie tells April that Emma and Isabel are sitting together when April visits the Fringe Cafe on the afternoon of the first day. When April asks Emma "what are you doing?" Emma responds and mentions Isabel saying: "staying here, what else? I'm meeting a friend later, but that's not till nine. We're waiting for Isabel and then we're gonna eat. But I guess you're not hungry."
We see an unnamed woman leave while she talks to Charlie and, as she is not there on the couch when April arrives, it may be her. Although Emma mentions Isabel earlier so it suspected that this woman is not Isabel, unless she left and to do something and was coming back.
The same woman walks past April when she arrives at the Watertown Bridge station.