User blog comment:12th Supernova/Chapter 789: The Final Countdown/@comment-5239749-20150606041657/@comment-10543621-20150608030030

There's a reason why Vivi remains the only other character to have been promised a spot in the Strawhats even though she didn't end up permanantly joining.

Despite her not being a great fighter, her willpower, courage, resillence and mental strength will always leave her a place among the Strawhats. That is something that has and still sets Vivi apart from other 'princess' type characters that the Strawhats have assisted over their journey.