Danbooru

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cirno-nee

チル姉

An older version of Cirno on pixiv known as Cirno-nee [チル姉] usually depicted wearing a yellow or orange scarf and a more mature demeanor, and sometimes has red-blue heterochromia. This version of Cirno appears on good terms with Remilia Scarlet and the other residents of the mansion.

(taken from Cirno Wiki)

Ricia(リーチャ) popularized the use of Cirno-nee with his Video manga series on NicoNico Douga.
http://www.pixiv.net/member.php?id=226209

As far as Ricia's Doujin goes, Cirno-nee is on really close terms with Remilia Scarlet as well as the rest of the Scarlet Mansion. Remilia and the Scarlet Mansion is also on friendly terms with the Makai residents such as Shinki and Yumeko. Sakuya deeply looks up to Cirno-nee as a sort of big sister character. While Sakuya-chan is still a child, everyone pretty much dotes on her causing her to be a little spoiled.

Other than wearing her traditional canon clothes along with just a yellow Scarf, Cirno-nee is also sometimes wearing a ⑨ hoodie and wearing her hair in an up pony-tail.

This tag implicates cirno (learn more).