I sincerely regret the time that I just wasted reading this manga. I will never recover it, nor the braincells lost by consequence of the sheer stupidity I've just witnessed.
With that being said: fuck Katie. And fuck stories where people do horrible shit to their friends but suffer absolutely no consequences and are treated as though they have zero accountability for their actions. Her motivations weren't even remotely solid enough to justify her betrayal and Denys is a pathetic doormat for smiling away her despicable treatment of him.
This entire story arc and Katie's role in it gave me traumatic flashbacks to the last two light novels of High School Prodigies Have it Easy, Even in Another World (although in her defense she didn't achieve the level of casually-forgiven, wanton, Machiavellian evil that occurred in that series.) and now I need to drink to wash away the frustration I feel from having read this nonsense.