Diane Elizabeth Lewis is the third wife of Ron Swanson (although his fourth marriage, as Ron married Tammy 2 twice), as well as the mother of two young girls from a prior relationship and a son named John, her child with Ron. She is a middle school vice principal, and has stated she "deals with hormonal psychopaths all of the time". She is not typical of the Tammys Ron Swanson has dated in the past.
Diane meets Ron Swanson after she repeatedly calls the new 3-1-1 hotline in the episode "How a Bill Becomes a Law" set up by the Parks and Recreation Department to assist the public. After receiving no assistance to fix the pothole in front of her house after numerous calls to the hotline, Ron decides to go (alongside Andy) to Diane's house himself to fix the pothole. She is impressed by his work, and asks him to show her so that she could fix the next pothole.
Ron struggles during the early parts of his relationship with Diane, because he is not very good with kids. After he fixes the pothole, along with Andy Dwyer's "help", he is forced to play princess with Ivy and Zoey, Diane's daughters, which includes getting his make-up done and having a tea party. Ron does not tolerate this, especially when he realizes it makes Diane laugh. When Diane asks Ron to stay for a little bit, Ron rebuffs her and leaves immediately. Andy, who can tell that the two like each other, leaves Diane a message in chalk on the street telling her to ask Ron out. Diane does so, and the two begin a relationship.
In "Halloween Surprise", Ron is in a relationship with Diane, as he has begun to really fall for her. He thinks that his life experience will be expanded by dating a woman who has two children and is nothing like the Tammys.
During the episode "Ron and Diane", Diane is very patient while Tammy 2 throws herself at Ron during a woodworking event. Instead, she shows immense apprehension, expressing to Leslie that Tammy is not the biggest threat to Ron. Diane explains that Leslie knows everything about Ron, including his past sexual exploits, and that if anyone is a danger to her and Ron's relationship, it's Leslie. Ron alleviates Diane's concerns by sharing with her his secret identity of Duke Silver.
In the season finale "Are You Better Off?", we find out that Diane is pregnant with Ron as the father.
In the season premiere, she and Ron marry and she jokingly says her middle name is 'Tammy'.
In the episode "The Wall", it is revealed that she has given birth to a baby boy named John when Ron takes the child to work, surprising his co-workers who didn't know about the birth.
Diane and the girls attended the Unity Concert along with Ron, and Diane encouraged him to perform as Duke Silver. She had a brief, slightly tense encounter with Tammy 2, but she maintained her composure.