I guess her almost witching out sort of makes sense given her wish was to be the one to destroy the city. I don't have her in my account so I only know what I've read so I guess I'll take your word for it.
I just hit a sidestory in the game where Ayumu invites Kasumi to a dessert buffet, and boy does Kasumi get excited once she knows you are, er, Anata/Yuu is coming also. Then she gets all jealous that Ayumu and Yuu went to a different sweets buffet when they were kids. It's pretty adorable.
(Also, Kasumi is so bad at being the scheming one.)