Say what you will about this guy. Without him this manhwa wouldn't be half as interesting as it is now.
His twisted ugly feelings for Pen could have made him Eckles level psycho but his head kept him on the "right path" even if it meant killing parts of him with Pen. The feeling meter sometimes feels like a reflection of his feelings to him not for Pen as the MC but a reflection of how he feels about his feelings for Pen.
I need a Derrek POV just to see him suffer through his anguish when he realises just what exactly he wants of Pen. To face his his ugliness instead of shirking away from them in guise of honour when in actuality it's cowardice. His guilt from when he lost Yvonne and his misguided attraction towards Pen always stopped him from seeing Pen as his sister, adoptive, though, it may be. His lashing out on Penn, telling her to not appear in front of him, cringed at hearing ogPen call him brother to Pens indifference when she calls him the Lord. It's all obviously sad. Something no rationale could justify or empathize to. Being the logical man he is this is even harder for him to accept that what he feels for Pen isn't petty dislike for stealing his sister's place but his loathsome lust placed on a misfortune girl.
What can I say, I love a tormented mentally ill boy