This is the relationship/rivalry between Erica Reyes and Emery Whitehill.
The relationship started off very friendly, as the two often agreed with the other in conversations. Emery even admitted to being jealous of how sexy Erica looked, though Erica admitted that Emery was quite lovely, making her blush.
In Jagged Memory, Emery developed romantic feelings for Erica, not realizing that she was a Succubus. However, when she went to tell Erica, she saw her with Seth. When Erica approached her to talk to her, she pushed her back, angry at her for "cheating" and confesses her love for Erica.
After months of accepting that Erica had no feelings for her, Erica arrived at her house unannounced, asking Emery to forgive her. Emery evidently couldn't stay mad forever, as she let her in and forgave her. After watching a movie together, Erica admitted to liking Emery as well. The two consummate their feelings for one another and have sex.
They are respectively played by Gage Golightly and Aimee Teegarden.