The midboss of Stage 1, Cirno says hello with a small flurry of icicles. Her power over cold seems rather weak today. Maybe it's because everyone is used to the winter chill by now and is dressed for the occasion.
The boss of Stage 1, Letty is a youkai of the winter season, and is happy but slightly troubled to see winter lasting as long as it has. As a youkai that can only exist during winter, she appears to you as a possible mastermind behind the delay of spring. Her attacks consist of snowball barrages and icy winds.
The boss of Stage 2, Chen is the acting guardian of Mayohiga, and attempts to chase you away, even though you only ended up there because you got lost in the snow. As a nekomata, she has cat-like speed and agility, which she uses to misdirect you in battle. She also seems to like spinning a lot. And she's the mid-boss of the extra stage too.
The boss of Stage 3, Alice is actually part of the magician race, whereas Marisa is a magician by occupation. She is a doll collector and although her dolls seem to be living they are in fact being managed by her with strings of magic.
The boss of Stage 5, Youmu is the half-human, half-ghost gardener of Hakugyokurou. She is the one tasked with gathering enough spring so that her master, Yuyuko Saigyouji, can unseal the body underneath the Saigyou Ayakasi. She is also the fencing teacher of Yuyuko. Her attack style is focused on using her swords to assault from all directions.
The boss of the Final Stage, Yuyuko is the Master of Hakugyokurou. She is the true mastermind behind the ongoing winter and seeks to take the spring that the player has gathered. With the ability to invoke death in living beings, she's a formidable opponent.