He comes to care for her. She’s the first person to give him love. The duchy isn’t known for giving love to one’s family. Even in the original they were all cold towards to each other. There were no love marriages. Only ones for the sake of connection and having heirs. That’s why neither the duke nor Ein show a hint of sadness or mourn the death of the previous Duchess. But Fl she’s the first person to give them love and they want to keep it to themselves, even going as far as to fight between themselves for her love.
This a main trope in every story between two characters. When a neglected/touch starved character receive love from another character, they latch on to it and want to possess it for themselves. This is common in villainess stories, when their abused/neglected by their family, their love interest shows a hint of love or affection towards them, makes them becomes the evil Villains when the love interest fall in love with someone else.
U don’t have to worry, Ein isn’t in love with Roselia. He’s interested in her cause she suddenly being nice and affectionate towards. He just wants to see how this plays out, but in the end he comes to love her as a mother he never had.
chapter 7 in does the son like his mother or am i trippin