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Hi! Thanks for asking- Spoiler alert below, read at your own discretion. ---

I assume Chapter Ten may be the reason preventing you from obtaining Duke's final ability. There are two paths in Chapter Ten- if you side with Irma frequently, you will land on the undesirable path where it negatively affect everyone's bond and friendship points involved. To avoid that undesirable path- Always side with Duke, continue avoiding Phylou's (Charos) wrath like "Fly!", and then side with Duke/Gary Combo attacks. (Using Sheila's Foresight will give you that bonus points, so do that too). At the end, you should land on the desirable path where it gives bonds and friendship points to everyone else involved.

Other than Chapter Ten, from Prologue to Chapter Nine- Remain loyal and truthful to your friends especially Duke and Irma. Be straight to the point. None of that hiding secrets stuff. Reasonable doubt is good, but does not benefit Duke and Irma both.  Be sassy, Duke really love that! For difficult choices such as this scene "Don't do it" and "No harm done...?", choose the "No harm done...?". Then coming across the option, choose "Pass the hot potato". You should always choose to speak up whenever Duke give you that opportunity, he appreciates someone taking the lead. So choose "I did" instead of "I should let Duke speak". The most important thing about Duke's route is that you should at least get closer to Rubio including the sex on Day One (Prologue). That's the way to get into Duke's vulnerable side. Not choosing Duke for Fox Den barely have any impact on the entire storyline, so no worries about that.

To summarize, you need more than 18 Bond points with Duke to get that final ability. You can still miss one or two opportunities with Duke from Prologue-Chapter Nine, but you need to land the most optimal route for Duke on Chapter Ten during the battle in order to get his final ability. I hope that helps! ^_^