Cirno (チルノ Chiruno) is an ice fairy who first appeared on the second stage of Embodiment of Scarlet Devil. As a fairy, she is exceptionally strong for her kind, matching the power of some weak youkai. However, compared to most of Gensokyo's residents, she's quite weak. Her personality is childish and she is sometimes considered idiotic by other characters. She stars as the main protagonist of Fairy Wars and, to a lesser extant, Touhou Hisoutensoku and Hidden Star in Four Seasons
Cirno first appeared as Stage 2 Boss in Embodiment of Scarlet Devil, then again as the Stage 1 midboss in Perfect Cherry Blossom. She then again later appears as one of the many playable characters in Phantasmagoria of Flower View and again in Touhou Hisoutensoku. She made an appearance as a background character in Hopeless Masquerade and then appeared as the Stage 1 midboss in Double Dealing Character. She also was a Day 1 Boss in Impossible Spell Card and later became a playable character in Hidden Star in Four Seasons.
While her age is unknown, she is one of the only characters in Phantasmagoria of Flower View that realized what was going on had happened sixty years prior as well. This implies that she has been around for at least sixty years.
Cirno is a childish, confident fairy who appears to have a superiority complex in regards to others. She proclaims herself to be "the strongest" on three separate occasions in Phantasmagoria of Flower View. In the game's manual, she is labeled as "9. baka" (⑨. バカ, lit. "9. Idiot") in an explanation of the game screen's layout. She was again labeled as such in the manual for Touhou Hisoutensoku. This implies that Cirno may be some kind of idiot, which does seem to be the case on some occasions - in Oriental Sacred Place, she is quite confused as to why nobody wants to buy her water-flavored shaved ice.
Despite this, she is smart enough to know when she is in deep trouble, such as her battle against Yuuka in Phantasmagoria of Flower View's story mode. She can also read, unlike some of her peers, such as Mystia Lorelei. She can also count (she shows knowledge of rounding and fractions in Fairy Wars).
Among other things, she greatly enjoys using her abilities to freeze frogs, then watch it revive while thawing out in the water. However, about one in every three attempts fails, shattering the frog to pieces. This has earned her a warning from Reimu Hakurei, who cautioned that she could be set upon by Suwako Moriya for her actions.
Cirno has the ability to manipulate cold to the point of being able to freeze water. However, even while saying "manipulate", when coldness is in an omitted state, it seems like her surroundings are chilly even in summer. In battle, she uses this ability to attack others with icicles, huge ice cubes, or wieldable weapons like swords, but also to freeze her opponents or their danmaku. However, despite being able to freeze something hot like flaming danmaku, she is unable to freeze lasers.
How she exactly manipulates cold is unclear, but Reisen Udongein Inaba, in one of her quotes in Touhou Hisoutensoku, speculates that when Cirno does it, she warms up inside as a result. This would mean that she just absorbs the heat in the surroundings instead of "creating" cold from nothing.
In the games as she generally only shows up as stage 1~2 bosses, she isn't that strong. In particular, fairies have an existence below that of humans. However, among fairies she possesses unusually strong power. She was playable in Phantasmagoria of Flower View and was able to defeat Reimu Hakurei, Marisa Kirisame, Sakuya Izayoi, etc. Technically when she "defeats" Aya Shameimaru and Yuuka Kazami, she didn't really defeat them as they still appeared unhurt after battle, implying that it was Cirno who ran away. In Fairy Wars, even though she wins when she battled Marisa, the one whose illustration showed to be "defeated" was Cirno; it was a simple case of Marisa letting her guard down.
In Perfect Memento in Strict Sense, according to the other fairies, while such things are written as "if she schemes, she is caught, so it is good even to refresh oneself of one's habitual anger," among it, there does exist the warning "if you meet her, you should flee." Furthermore, Eiki Shiki, Yamaxanadu commented on her saying "although you are a fairy, you possess great power". Since she goes beyond the limits of fairies, there may be the possibility of dying.
If humans were to fight her gracefully, it would seem that the weapon would become frozen and it wouldn't be possible to win to begin with, but if we were to just run away, if we poke at the point that she is an idiot, we are able to run away immediately, since she is not able to answer even simple riddles.
Her known name is "Cirno"/"Chiruno" (チルノ), which has a currently unknown origin. It's likely based on the Japanese pronunciation of the English word "chill", chiru (チル) . Her name appearing as "Chilno" in Eastern and Little Nature Deity supports this possible origin. The earliest known official romanization of "チルノ" is in her Phantasmagoria of Flower View profile as "Cirno".
Her design in Embodiment of Scarlet Devil, Perfect Cherry Blossom and Double Dealing Character shows that Cirno has aqua-colored eyes and hair. She wears a blue ribbon, light pink blouse, blue jumper dress and seems to be wearing white socks. She also has icicle-shaped wings. In Phantasmagoria of Flower View and Touhou Hisoutensoku's official art, her ribbon is green instead of blue and her sprite in Phantasmagoria of Flower View still keeps the blue ribbon. In Fairy Wars, her bow is once again blue. Her blouse has been changed to white and she seems to have bigger eyebrows (because she was drawn by Makoto Hirasaka). It's been said that Cirno is one of the easiest designs ZUN has done.
Her alternative outfit in Phantasmagoria of Flower View is same as her normal outfit, except her bow is a darker shade of green, her blouse appears light purple, and her dress is purple as well.
Her appearance in Hidden Star in Four Seasons gives her tanned skin, a flowery vine wrapped around her body, and a sunflower ribbon around her neck.