The duke's story starts on chapter like 35-ish. The partner is the attendant (Baron, promiscuous, head of the paladins, dude you see on all fours being tormented by the duke, blue eyes and long hair). In chronological order, the baron's mother (baroness) tried to kill the duke and the brother (current pope now) when they were kids because their parents died and if they died as well then the lands and stuff would get transferred to them or something. The baron was a child at the time and he gave them poisoned cookies but regretted it and gave them the antidote. The duke knew all this and pardoned his and the baroness' lived if he gave up his right of succession and became his attendant. The lands and everything was sold and his mother was placed in house arrest. The husband and older brother were killed. For years the baron is the duke's servant, being called the duke's dog. He sends money to his mother who resents him and pressures him to kill the duke but at some point the baron falls in love with him. Then one day a count gives the duke wine with aphrodisiac, the duke gives it to the baron thinking it's poison just to mess with him and find out which type of poison it was but they end up doing it. The duke was about to kill the baron when they woke up but the baron begged for his life and instead made him cut his hair. Then they start investigating the count. The count had an argument with the baron and then men arrive to kill the baron. He kills them all but ends up stabbed. The duke rescues him but blames him for getting injured and sleeps with him again, worsening his state. (The duke is the uke btw). The baron couldn't tell him that he believed it was the count who send the men after the duke. He ends up waking up a week later and runs to the duke thinking there were still assassins. They were. The baron rescues the duke. The baron tells him they were sent by the count but the duke says they were sent by the baron's mother. When the baron went to see her she had already unalived herself. Turns out it was all planned by the duke who didn't like that the baron's mother spent his money and blamed him for everything so to get rid of her, he sent her the baron's hair saying he had been tortured. The mother then made the count borrow money to hire assassins. When the baron found it out by the duke himself he asks if the duke had ever loved him, to which he says he doesn't. He just wanted the baron for himself and wanted to get rid of everyone around the baron. He says he knows the baron loves him and won't betray him. The baron feels hurt and says he'll leave him but the duke laughs and tells him that tomorrow they'll go to get the money back from the count. The next day the baron is there, implying that he wasn't able to leave the duke and the duke just laughs. And yup that's literally how it ends, no side stories or anything. The duke sucks. To explain his twisted personality, his parents died when he was a kid so he became that way to protect his brother. He still sucks though.
Otto's (pregnant dude) story is really cute. The duke's story is much darker, not satisfying at all if you're into wholesomeness. The duke as you can see is cruel and twisted. He doesn't change and instead breaks his partner. I wish I hadn't read it.