While she is the architect of daddy's harem without doubt, I don't think she is a plain character at all. But I agree with you in that it will be nice to see more into her psyche. We have seen quite a lot about Abigail and Lilith, but Becca remains more of a mystery.
In the end, though, this is a slice of life harem game, so her endgame need not be anything more than to be happy, which is in some sense everybody's ultimate game in pretty much any setting. Being happy seems to be for her to be with the ones she loves: her two sisters and her daddy. I think it is a totally understandable and worthy objective to have.