She is jealous over Nomura (sensing Yuki affection for him) and Miss Mizutani (misunderstanding her real intentions towards Masato). In chapter 18, it turns out "Maa-kun" is actually Kaoru.
She harbors jealousy over Nomura (assuming Yuki's feelings are directed at him) and Miss Mizutani (misinterpreting her true feelings toward Masato). In chapter 18, it is revealed that "Maa-kun" is actually Kaoru.