Seems like a kinda beta MC but in reality the book simply is moving to slow for the plot to not seem beta'ish so the author simply needs more experience and practice.
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LIKEYou have to know his sister is one of the four leaders of current Devils, so she is hella strong. He used multiple ways to avoid her but she brute forced all of them. And in latest chapter, between their talk, Savion already said that Serafall gave her everything to him. Almost all of her love, her attention and her time. It is not beta'ish to go along with the whims of someone who gave her everything. All in all, Savion gave up resisting because of Serafall's love. Her love is too big for him to handle but he is working hard to receive all of his Nee-sama's love properly.