i dont know! i called her yuki for ages but when i read the Bücher it sagte yuuki. sooooooooo i guess it dosent realy matter. Du just have to decide what Du like better.
I think it's to differentiate between the kanji for "snow" and the kanji for "gentle princess", as in Yuuki's name. Both are pronounced identically but are written with different characters. In English translations, "snow" is written as Yuki while "gentle princess" is written as Yuuki.