Shirahoshi is maybe the most politely/formally-spoken character to show up in this series so far. She uses the extremely polite わたくし/watakushi as her personal pronoun all the time- even Brook, the only other character to use it thus far, only uses it in particularly formal situations such as introducing himself.
She uses polite verb forms, and is very polite when addressing others even when clearly scared out of her mind. She also uses a lot of sonkeigo (the set of vocabulary specifically designated to be more honorific/respectful- it’s pretty obtuse nowadays and rarely used in modern Japan, mostly in set phrases like gozaimasu).
For example, she asks who Luffy is with どちら様でいらっしゃるんですか/dochira-sama de irassharundesuka?, which is all sonkeigo. どちら様/dochira-sama is such a formal way of asking ‘who’ that I had to look it up to make sure that’s what it meant, and いらっしゃる/irassharu is the honorific way of saying ‘to come/go’ that nowadays is only really used in certain set situations like welcoming customers into a store.
Shirahoshi also calls everybody by the highly respectful honorific -sama, which isn’t that surprising given her very polite speech patterns. What is surprising is that she goes so far as to even use it for Vander Decken, the guy trying to kill or forcibly marry her.