Shirahoshi might not feel like she has much agency in the plot, but at least her theming is consistent and she succeeds at what she endeavors to do without compromising.
Rebecca, who manages to remain merely a damsel in distress despite styling herself as a fighter and a combatant, really is just the very most pathetic.
Lets not forget the lack of self-awareness she has for her situation:
1. She opposes Doflamingo, yet she's playing right into his scheme by entering the colosseum.
2. She fights in the colosseum, yet her goal of the mera mera fruit, which she'd use to start a rebellion, wouldn't even be enough to defeat her oppressor if she did win.
3. Yet she's so focused on this side-plot trap, that she's willing to slit the throat of any of the would-be allies she comes across (Luffy being an example but literally all of the colosseum would oppose Doffy in the end) in the name of a slightly better chance inside the ring.
Nominally standing for freedom, yet in actuality being manipulated into being nothing more than your sworn enemy's puppet, is basically the crux of Rebecca's entire character. The finale where that literally happens is merely the one point where Oda spells it out for us.
Rebecca is unbelievably pitiful.