@Jtg1111111 and @Pluton9
Following the narrative timeline (the one of the publications) at the time of SO2's publication, Avenger didn't exist; just look at Ais's status sheet published in SO1 and you'll see that in the skills section there are question marks. It's Omori's way of indicating that Ais has some skill but it's not available at the moment.
What's the point of an author hiding a character's skill if literally in the next publication said skill is already going to be available? Why hide it then?
Many times authors hide skills because the revelation of them will serve for some big plot twist or development, but was it worth hiding Avenger for that effect? This skill consists of a status increase, as are many others such as Ariel herself, so its revelation wouldn't serve to create some big plot around it.
So why did Omori hide it? It can only be because giving Ais 2 gigantic boosts from the beginning of the series wasn't very convenient for the development of the plot, as in the case of the aforementioned battle, if she had used both Ariel and Avenger she would have easily won against Udaeus and to hell with her level increase, but that couldn't be done like that because a level 6 Ais was needed...
Pluton, you say that Ais can activate it unconsciously and that no one can prove that it wasn't like that, but if the ability itself didn't exist yet, much less its characteristics. How can you think that something that was introduced just for SO10 years later is useful to analyze something from SO2?
Proof that such an implication is unfounded was, as I mentioned in the first post on this topic and which in fact jtg111 also mentioned although for some reason he used it to go in the contradictory direction, is that against Asterius the narrator had to expressly tell us that Ais was using Avenger, because by deduction it cannot be assured that this is the case, as you two are doing against Udaeus.