Hmm, I don't think that Arthur has necessarily to become weak. He still has his own principles and goals he wants to achieve and he knows that he needs a clear mind for that.
As for him being romantically involved with any of the female characters, I think (just a speculation of mine, I could be totally wrong) the author is showing us this because we don't know anything about Arthur's past life relationships. As far as I remember, we were never told if he had a love interest or a wife in his past life as a king. It maybe that these current events are shown to give Arthur some character development, and not only his physical strength and abilities development))
He was alone in his past life basically. No friends and all his family well, the person who took care of him, died when he was younger ( since he was an orphan I'm pretty sure ).
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I'm fully caught up in the novel and I think that her interactions with Arthur are just emotional buildup for the angst to come. It's not that important in a conventional way, but it is gonna be really impactful for when Arthur meets her after she turns into Cecilia... His whole relationship with Tess is really just buildup for the betrayal (〜￣△￣)〜
I might actually drop this because I loved this manga thinking there wasn’t going to be any annoying romance in it. Yea, I was fine with the girls showing some affection towards him, but now Tess is taking it too far. Apparently, Arthur will become weak and soft because of Tess which is why I might drop this, especially the fact Tess kissed him without consent. I like strong and sometimes op characters, but if my boy Arthur is about to become weak and soft because of some girl(s) that he won’t reject, Imma stop reading this. Also, before anyone attacks me about Tess and her feelings, this is my opinion and I know it’s normal for kids that are Tess’s age to have feelings for other people, but please respect my opinion. Thank you for your time (￣∇￣")