So, from what it seems, i'm guessing Aquila must have been the dragon from chapter 1, in some way, thus giving him superhuman abilities and strong magic- including the magic to reverse time. In a few of the panels, we see that Aquila is holding 2 rings (I am assuming these are wedding rings, since Lamia had one on during one of the scenes). Later on in the story, we learn that Lamia is supposed to become the duke's heir and be the next duke (duchess, rather), and Uriel was going to become a knight.
However, Uriel's hand gets destroyed, thus meaning he can no longer be a knight, and he and Lamia must swap roles- he becomes the heir, and Lamia will become a knight. So, what we know is that Aquila destroys Uriel's hand. It's Aquila's fault that Uriel couldn't become a knight. Since Lamia cares deeply about her family, she becomes furious at Aquila for doing such a thing to her younger brother... for destroying her family's legacy, basically. Now, here's the bigger question- we know Aquila has feelings for Lamia, so WHY would he do such a thing to her family? Well, this is what my theory has come to. Let me explain.
Basically, I'm guessing Aquila has the ability to turn back time. In the main timelines that he's in, he meets Lamia, who will become the duchess. She falls in love with him, and the two get married. Eventually, we are led to the gardening scene that was shown in this chapter. Lamia says something about "Spring coming soon". Then winter rolls around, and clearly, Lamia meets her fated end and dies before the spring. Aquila proceeds to reverse time. Each time he does so, he is looking for a way to save Lamia. My guess is Lamia dies yet another time, and once again, Aquila decided to reverse the time. However, he decides to try something different. He attempts to change the time again, thinking, "Will Lamia be able to live if she becomes a knight, instead of the duke's heir?" And so, he ruins Uriel's hand, changing their fates. What he didn't see coming would be Lamia's hatred following- since, in her eyes, he ruined her life and gave her 2 years of agony.