Cirno is an ice fairy. While she is described as a weak boss in the Touhou series, if we consider the power of average fairies - who appear often as regular enemies - we can certainly say she is exceptionally strong for her kind. Her personality is childish, as you might expect from her appearance.
During the events of Embodiment of Scarlet Devil, she attacked the player, who was just passing by, out of instinctive aggression. She was helplessly shot down.
In Perfect Cherry Blossom, she returned to take revenge for her previous fight, but was only a mid-boss and was easily defeated like one.
In Phantasmagoria of Flower View, Cirno aimlessly wandered about, apparently unconcerned by the flower incident for any reason other than the flowers being something else for her to freeze and play with. She really wasn't meaning to do anything but play games.
In Unthinkable Natural Law, Cirno was intrigued by what she believed to be the great Daidarabotchi and hoped to tame it so that she won't be afraid of anything anymore. She showed her intelligence by admitting that she is incapable of defeating the Daidarabotchi if it lives in the hot spring's boiler, since it is too hot for her.
After the events of Strange and Bright Nature Deity, in Fairy Wars, Cirno tries to find those responsible for destroying their house. After learning that it was the Three Mischievous Fairies, goes in search of those responsible, to which defeat easily, because Cirno is the strongest.
After, Cirno wanders around for a while, bored of weak opponents. While wandering, she encounters Marisa Kirisame, making Cirno think she is a worthy opponent. They battle, and later, Marisa pauses and realising how powerful Cirno was. Marisa goes slightly harder, but is still defeated. Marisa claims that she didn't try as an excuse for her loss.
She was last seen as the midboss of the first stage of Double Dealing Character.
Gains Close Quarters Combat when attacking or attacked by Infiltrators. Performs a follow-up attack and attacks are guaranteed to hit.
Ice Fairy - immune to Ice debuffs and attacks.
Flying - immune to ground attacks.
Baka - immune to psychic and fear attacks.
Ice Sign "Icicle Fall" - chance to perform a weak AoE attack when an ally attacks or is attacked
Cold Pressure - resistant to Bleeding and Burning.
L1 - Ice Beam
Ranged Ice Magic Energy
(enemy) Frozen - cannot dodge attacks.
(enemy) Deathfrost - takes magic damage and loses stamina each turn.
(enemy) Immobilized (50% chance) - cannot use Melee abilities.
L2 - Ice Sword
Ice Slashing Melee
(enemy) Tenderized - takes extra damage from slashing attacks
(enemy) Bleeding - takes damage each turn.
(enemy) Fumbling - single-target attacks provoke a counter-attack
L6 - Ice Shards
Ice Slashing Ranged
(special) Hemorrhaging attack - causes all applications of Bleeding to trigger
(enemy) Pin Cushion - takes extra damage from follow-up and counter attacks
(enemy) Pressure Points - causes either Weakened, Dizzy, Slowed, or Exposed.
(enemy) Winded - cannot perform follow-up attacks.
L9 - Absolute Freeze
3 round cooldown
(special) Coldsnap - always crits chilled targets
(special) Catastrophic - always hits and cannot be protected against
(special) Paragon Exploiter - does extra damage to enemies with Combo Setup, Stun, Dizzy, Weakened, Slowed, or Exposed.
(enemy) Exhausted - cannot take extra turns. Quick Actions become normal actions.
(self) EYE'S THE STRONGEST! - takes an extra turn next round with increased stats and immunity to Stun. Removes debuffs when applied.
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