Sadie met Finn when she was 15 and he was 16, and they've been together ever since. Finn stated that Sadie is the only girl he'd ever been with. Some time after, Sadie found a flat, furnished it and she and Finn moved in.
After the Storm hit, Sadie recived the power of Mind Control. She used her new power to make Finn the perfect boyfriend, as he was childish and inconsiderate before. When Finn realised Sadie was controlling him, he tied her up to the bed and held her captive until he could find "a cure" or a way to supress her power.
When Finn started his community service, Finn said that Sadie is the name of his dog, and that he needs to get out early and feed her. Rudy found Sadie in her bedroom and let her go, only for her to come back and give Finn a second chance. After she saw that he wasn't changing, she used her power over him once again, only this time he welcomed it - it is the price he has to pay to be with her. Jess, who couldn't stand seeing Finn losing his free will, stepped in and asked Seth to take Sadie's power away. After he did so, Sadie and Finn broke up.