Most of these are person A's unhealthy one-sided crushes on person B
In True Beauty Sujin (Seoho's childhood friend) grows to be possessive of him
In Light on Me Sohee has a one-sided crush on Daon but sort of almost acts as if they're already in a relationship, she's very possessive of him and rejects anyone showing interest in him behind his back
In My Lovely Liar Syaon is crushing on Doha, and she plays with his feelings and manipulates him in order to gain his attention, once she took advantage of his trauma and threatened to kill herself
In The Director Who Buys Me Dinner Denis feels like Dongbaek is the only person who truly understands him, therefore he becomes obsessive of him and extremely hostile towards Dongbaek's boyfriend Yudam
(Not sure if this counts) In Color Rush Yeonwoo begins to be obsessed and dependent on Yoohan, and has thoughts about locking him up etc, they end up together in a rather healthy way but only after a lot of ups & downs