Broadly speaking, there are two reasons why critics might think that Hellena is the rover who gives the play its title. The first is internal to the play: Hellena roves more widely from her assigned course than anyone else. Wilmore takes few social risks with his actions and attitudes; Hellena risks much. The second is historical/biographical; Hellena makes independent choices somewhat similar to Behn's own, and so one might think that Behn might make such a character the namesake of the play.
Because as wilmore wants to experience life and love before actually being turned into a nun critics think that that is the reason why she is the rover instead of Wilmore who is simply just a playboy