He actually is the hidden prince of the kingdom that get defeated recently, but thanks to being born illegimate, his father and royal brothers who love him protect him by pretend they keep him for being a genius swordman while delete all record involve his birth. Which result in they all got killed, while Eclis got enslaved and treat as a prized spoils of war.
Penelope fear him because she know he stay proud of his blood and never bow to the "enemy kingdom"' duke house.
But instead Eclis angry with jealousy that the royal house prince can have all the time with his master, even force her away when she was trying to spend time with him.
The matter is inferiority complex, so he destroy her position, so that she become fallen, the royal and duke house will discard the "commoner her", while he will gain recognise as swordmaster, such that he can take her as his own with pride. Because "as long as she is the ducal princess, he is the lowly slave knight, the royal bastard who stolen everything from him can steal her from him too, with the duke house helping them." Such is the brainwash was used to control him and start the betrayal.
Eclis do fight to protect Penelope at the end, and seemingly died with his body lost, because the prince remove the "corpse" from the field (trying to remove competitor), then the "Eclis's corpse" itself wake up and escape away. The prince know he is alive but cant say anything about it. Then Eclis recover on his own, lost memory, and by tracing back, protect Penelope due to linger feeling, but at last he find out that without his memory, he dont really want to involve with Penelope, and they give up both the good and bad past they once had. At least they are free from the hatred that once polluted their life. Eclis go away from Penelope.