These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
It Was His Sled: The ending is a Twist Ending but due to the popularity of the first OVA series, the ending is better known than the actual details and dynamics of Riki and Iason's relationship and how things got to that point.
Magnificent Bastard: Iason, both magnificent and an utter bastard. He got his feisty sex slave to love him and die with him.
Iason really needs to be noted for his magnificence here because from the get-go, how he confined Riki, where he put Riki's pet ring and how he used it was to condition Riki to want him around. That's right, condition. That even when Riki wanted nothing more than to get away from Iason, he just wouldn't be able to live a normal life thereafter without Iason in it. Even letting Riki go back to Guy for a year was part of Iason's plan to show Riki that his old life was over no matter what and was very effective in proving that. After sex with an elite like Iason, it's hard to go back to a normal person like Guy. Really hard.
Memetic Sex God: Iason, who can cause orgasms with his voice alone and undress you with his eyes.
Moral Event Horizon: Kirie intentionally crossed it when he sold out Guy but did Guy do so when he decided to cut off Riki's penis to remove the Pet ring?
The Scrappy: A few may count given the ambiguous morals of the people of Amoi but Kirie and Guy stand out to the fandom.
Uncanny Valley: It's easy to forget that Iason is an Artificial Human, untill he rips Guy's arm off. Not to mention the time he made his manhood grow bigger in an enraged state of lust.