@Monkey:
Robin wasn't operating within her group as the oldest child of a powerful centuries old dynasty.
She was a hired agent in a secret assassin group.
Excellent point lol. I realized the flaw in my "secret agenda" possibility a few minutes ago, but thought it was too late to edit that. And Robin is still a different situation despite the similar elements since she isn't as inherently tied to Crocodile as Reiju is with having literal blood ties to Judge and high social stature.
However, the possibility for Reiju to not stick it out as a complete antagonist is definitely there, although it isn't definite and she can also understandably be an antagonist. She's actually advocated the life of a royal compared to mundane pirate life to Sanji and doesn't find Judge's acts to be detestable at all. And she put the cuffs on Sanji without hesitation. However, she criticized Yonji's attitude of starting up pointless fights for the sake of it to the point of striking him in a comedic sibling spat fashion and apologized to the Straw Hats, who she shouldn't see as more than pathetic plebeians from how the rest of her family and even guys like Cracker feel about royalty status. And she even thanked the Straw Hats for taking care of Sanji, and is the only Vinsmoke that actually both acknowledges and compliments Sanji for getting stronger since he left the family. Not to mention keeping the Straw Hats' presence a secret, although Yonji claimed to agree due to not wanting to mess up the wedding plans (whether Reiju did for that point or purely as a favor to the Straw Hats is up in their, but either or both is possible).
So she's already complex in terms of where her ideologies lie compared to the other Vinsmokes. There hasn't been any vivid moments where she actually agrees or sympathizes with the Vinsmokes' dogma towards social supremacy, she just benefits/appreciates being a royal and works with them due to them being her family than actually being a staunch supporter of their views on treating lower classes and soldiers like trash (she even called Sanji a gentleman). Not that she necessarily has a problem with it either. I don't really see her having a 180 on her moral compass to hate Germa 66 or even have to fight back if their actions haven't bothered her thus far, but I can definitely see her being entertained by the Straw Hats enough to give them openings towards mucking up the wedding and turning a blind eye. Some type of situation where she isn't actively rebelling against Judge and is fine with the prospect of the marriage going through and the Straw Hats losing, but also not minding the thought of seeing the capabilities of the Straw Hats and what type of effect "a puny pirate crew" can have on a royal's fate. They don't necessarily have to be buddy-buddy, and Luffy still wants to exercise caution towards her despite being grateful. Those redemption examples I mentioned all happened after those characters got beaten up, renounced their corrupt ways, and helped the Straw Hats. But both the Straw Hats and Sanji fascinate her very much so and she seems to have legitimate respect for their ethical values, not just basic tolerance, which shouldn't be ignored.
Where this all leads will be very interesting. But I doubt that her meeting the Straw Hats that early on, having those interactions, and keeping it a secret will not be relevant later on in some way. I'd find it a bit wasteful if they still fought without any further insight on Reiju's situation and mindset in relation to assisting the Germa. Unless she just likes messing with people's hopes like a sociopath, but I don't see much evidence for that yet. Come to think of it, do the Vinsmokes (besides Reiju and Yonji) even know that the Straw Hats are around? Big Mom does and I don't see why she wouldn't inform them. Although I can see her not doing so from the perspective of wanting to take responsibility for the Straw Hats due to this being her island, and not wanting to promote any inconvenience towards Germa 66 as a marriage commodity. Like how a party host doesn't want to alert their guests that anything could mess up the party to potentially kill the mood or something like that, I hope it makes sense lol. Either way with how Reiju's character turns out, I can find it entertaining and believable. There's actually compelling stuff here to analyze and debate about than the likes of Monet.