The protagonist forces a love interest who is already in a relationship with a third party into a physical relationship, and the love interest comes to like it over time, thus either cheating or breaking up with their original partner. If the protagonist is the one having their partner seduced, use 'Netorare' instead. If there's no evolution from unwillingness to infatuation, use 'Adultery' instead.
Note: There's no magic in this novel. This was previously in the Realistic Fiction genre but it's not included in the choices. So I picked the closest one to it. Onoda Ruki is just your ordinary high school student. He strived for being the Classmate A who's unimportant to the story. Despite being the Classmate A. Onoda has a secret desire which he always had ever since young and that was to steal each and every girl from their guy. Join him as he entered his high school days as he conquer and steal every girl he sets his eyes on. And along the way, the growth in his character and those around him.
I fell in love. Her name was Shirasaki Yukino. I could only look at her from a distance. However one day I heard that she already had a boyfriend. Even if I love her, I can’t have her. The devil sent me an invitation. I can make her mine!
This is the story of a young slave gladiator in an underground arena. He doesn’t know about his past or how he got to where he is, only that his name is Aegir, and that he is strong. One day he kills the boss and escapes the arena, joining a band of mercenaries as a new recruit. During one mission, they encounter the vampire, Lucy, who slaughters the band with her inhuman strength. After learning that Aegir only knows how to kill, Lucy lets him stay at her house, educating and taking care of him. Two years pass, and on the day of Aegir’s departure, they both exchange a promise that if Aegir becomes a king and owns the land of Erg forest, he can come and take her as his woman. Making this his life goal, Aegir set out on a journey to become a hero, a king, and found his own kingdom.