I can appreciate the fact that the actual training itself was limited to a single chapter. This certainly didn't need to be drawn out and could've very well been accomplished in one chapter which was the case.
I'm also pretty sure that the sister's ability is Za Warudo!!! She even has an accompanying pose when she executes the technique. She low key probably reads a lot of JoJo. hahah
Lastly, MC can't really complain when the rewards are that good, now can he?
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"When there is evil in this world that justice cannot defeat, would you taint your hands with evil to defeat evil? Or would you remain steadfast and righteous even if it means surrendering to evil?" - Lelouch vi Britannia as Zero.