Mary is Leonard's single mother and 4th-grade teacher. She comes up with these crazy ideas for her class and knows how to get her students to listen. She is loving, caring, overprotective, hilarious, and flirty. She has a New York accent. Let's just say she's the teacher you'd love to have in your class. She also really loves her boys, Leonard and Scott and is very protective of them.
Mary Lou is a tall and attractive woman with Caucasian skin, curves, and orange hair. Her outfit is a green jacket, skinny red pants, a yellow shirt, and black shoes.
Mary becomes Leonard's new teacher after earning a teaching degree. She buys him a purple bunny backpack but Leonard angrily turns it down because he wants to be normal.
At school, she meets Scott Leadready II unaware that he's really her dog. And Scott instantly becomes the most popular and Leonard becomes envious. When Leonard finds out the truth, he, Scott, and Ian decide to run for president. And Ian wins by 1 vote.
In one episode, she was sick and a substitute teacher had to take her place and all the students liked her except Scott due to his canine super sense.
At the end of the year, she gets nominated for a teacher's award and the principal lends her his RV, only no pets are allowed. So they leave Spot, Pretty Boy, and Jolly at home.
At a gas station, she meets Scott and he convinces her to take him along (along with his "family"). When they arrive, she leaves the boys at the beach, who later go to Dr. Ivan Krank to make Spot's dream come true.
Back at the RV, Scott (who's now a grown man) and Leonard forget how Scott is a man. She invites the human Spot for dinner and falls in love with him. After a long fight with Leonard, Spot leaves the Helpermans forever. The next day, Mary goes to the awards ceremony while Leonard is in bed crying.
After Spot is turned back to a dog, Leonard is at the awards ceremony and she has won. Spot, Pretty Boy, and Jolly are outside cheering for her. That night, Spot is dressed up as Scott and convinces Mary to take him home with them and Jolly and Pretty Boy are hiding under the RV's bed as part of Scott's plan.
- Her full name is a reference to Wendy Darling from Peter Pan.
- Mary Lou was originally going to have a more suggestive design, with tighter pants and more buxom attire, but Disney would not allow this.
- In "The Turkey That Came to Dinner" we learn her full name. Also, Angela-Darling might've been her last name before she married Leonard's disappeared father.