Felix knew that he was taking out some of his frustrations by demanding a spar, but he couldn't help what he had seen a few times in the Duke's Tower that was unexplainable to him.
There were times he thought that Agnes wanted to pay a little more attention to Iro than she wanted to pay attention to him. Even though he knew that she wouldn't be romantically interested in a boy so much younger than her, he couldn't help the jealousy that ate at him whenever he experienced it.
Not only that, but the situation where Iro didn't even want the attention of Rose and brushed it off as if it were nothing rubbed him the wrong way. In his mind, it was inconceivable that the other apprentice would be immune to the girl's advances, particularly the cute noble girls who were openly showing their favor.
Even when Felix was twelve, he remembered how nervous girls made him. Iro was so unusual and it bothered him for a variety of reasons.