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.)