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.
In chapter 18, she discovers that "Maa-kun" is in fact Kaoru. She feels envious of Nomura (believing Yuki has affection for him) and Miss Mizutani (misunderstanding her intentions towards Masato).