Yep, it's Mokou. I was thinking of her when I made that haiku.
Not that easy: I thought immediately of Meiling, but that seems off for some reason.
Imma say Satori.
Half-serious guess: Cirno? (She's weak, but she considers herself "the strongest...")
YuugenMagan?
Koishi?
Nue.
Yukari?
Yuyuko.
Gengetsu
Elly.
Reisen.
The Giant Catfish.