@Southpaw "But sure, once again.. Just for the simpicity.. Lets say that your version did happen. "
It's literally what happened, the scene wasn't really cut as much as they were talking as they moved about. It's easy to see from the whole scene(including the pages before) that Iris was indeed paying attention up until the other girls asked about an explanation on what Mathias was saying about the pass. It was then that Iris left. Easy to see that she was bored with a conversation she didn't enjoy, and was instead interested in the buterfly. Why is this behavior acceptable? THe reasoning lies below. In my next answer.
"I don't even think you understand the concept of 'gross negligence'..."
No, I don't think that YOU do.
Gross Negligence can only happen when someone doesn't fulfill their responsibilities with due commitment, which may result in problems.
Which is not the case, was never the case with Iris. It's not her responsibility to formulate plans, or be involved in more complex actions. She is the powerhouse of the group, her only responsibility lies in destroying whatever it is that Mathias tells her to and fight enemies head on, and not once did she fail to do exactly that.
To put things in perspective let us convert the scene to D&D. IN place of Iris we'll have a FIghter, in Place of Mathias we'll have a Mystic Theurge(a Prestige class between mage and cleric), and instead of Ruri and Alma, we'll have a Rogue and a Ranger both trying to Multiclass with Wizard. The Mystic, Fighter, Rogue and Ranger go through the checkpoint, the Mystic starts talking about the pass they received at checkpoint, the Rogue and Ranger, who are learning magic from the Mystic ask more details to the Mystic.
Do you really think the Warrior, the only one on the team who doesn't care about the minute details on magic, would remain in the conversation?
He knows the pass is enchanted and my cause problems, he heard about the spell, and thats ok for him, why should he be there to listen how the spell is formed if he'll never be able to use it and no one expects him to?
Also Mathias doesn't have to explain anything to her because she has centuries of experiences of her own, fighting demons, humans or whatever threat to her territory.
Which is why she has more freedom to do what she wants. Unlike the other two who are essentially Mathias apprentices.
The reason why she is likeable is the contrast between her actions and her actual performance, she acts curious and childish because everything she is currently experiencing is new to her, but she is extremely dependable when need be. She's earned her carefree atitude.