Samantha Louise "Lou" Fleming is Amy's older sister. At the beginning of the series, she moves back home from her job in New York to help get Heartland back running again after her mother's death. She first has a New York boyfriend (Carl). She later gets together with the veterinarian Scott Cardinal. Eventually, she meets her husband, Peter, thanks to Lisa. Their relationship is secret at first because Lou knows Jack won't approve to him due to his job in the oil industry. The couple marries at the end of season 3. They move to Dubai, but Lou soon feels very alone. She grows to miss Heartland and her family. When Lou and Peter move back home, Lou finds out she is pregnant with her daughter, Catherine. She later gives birth to a baby girl, whose name is Catherine Marion Minnie "Katie" Fleming-Morris. Amy Fleming and Ty Borden deliver the baby. She later adopts a foster child named Georgina "Georgie" Crawley.
Dealing with the loss of her mother after she died in a car wreck, Lou moves back to Heartland from New York to help bring the ranch out of financial troubles, as she solves them she does seem to frustrate the others. When her boyfriend from New York comes to visit (Carl),he proposes to her but later she finds out it was only for his job. Carl does deny it but she does not believe him. Lou also begins dating the ranch veterinarian, Scott Cardinal but they later break up. Later the series, Lisa sets Lou up with Peter, an oilman and eventually Lou and Peter get married, have their daughter Katie and later officially adopt another daughter, Georgie. Maggie's Diner is up for sale. Lou fights to have Maggie's Diner turned into a heritage site to keep it from becoming a chain food store. After it's declared a heritage site, she purchases it with the help of her father, Tim. An old friend of Lou's becomes the manager of the diner.
Dr. Scott Cardinal - The two rekindle their romance after Lou returns from New York. They break up in season 2.
Constable Mark Rodriguez - Lou began a relationship with Mark after he brings his horse to Heartland to have Amy work with him to get over his fear of water. Just as Lou was starting to open her heart to Mark she ran into him in town kissing another girl.
Peter Morris - After Lisa gave Lou's email address to a friend of her friend who passed it on to her son unknowingly, the son, Peter contacted Lou on a whim of fate. Lou eventually replied to try online dating and start getting over her experiences with both Scott & Mark. They scheduled their first meet up before Lou realized she had seen him in person before. He is known as the oil man who she went head to head with when his company Bedford Oil was drilling on Jack's property. After she found out who he was, she wasn't interested in dating him, but he pushed her to give it a try. They kept it secret initially. They eventually married and have two daughters, Katie and Georgie. They adopted Georgie during season 6, after Jack was turned down to adopt her because of his age. Later on they decide to separate and eventually get a divorce.
Mitch and Lou hit it off with an awful online date and continue to dislike each other for a while but eventually become romantically involved. They started dating officially at the end of season 10. They break up in the Season 10 finale due to Mitch thinking that Lou and Peter went to hook up when they were really trying to help Amy. After he catches the divorced couple together, Mitch packs up his trailer and leaves. They get back together in Season 11 but their relationship doesn't last too long when Mitch breaks up with Lou because she's always going to be in New York and they're not going to spend any time together.