Lonnie, like her sister Connie, enjoys teasing Bonnie, the youngest of the three, and, like Connie, looks down on her. According to her, she got the "looks," Connie got the "brains," and Bonnie got "the rest." She seems to act snobby and rude, not only to Bonnie, but to others like Kim Possible, as seen when Kim tried to introduce herself to Bonnie's sisters. When Bonnie introduced Kim to her sisters, she said that they're "older, way older," which left them both insulted.
Bonnie is Lonnie's younger sister, but she enjoys bullying her alongside their sister, Connie. They clearly have a poor relationship, as Bonnie insulted their age while introducing them to Kim
Lonnie appears to get on well with Connie, who she was seen with in "Bonding". As she was seen only once in person, it is not known if she hangs around with Connie all the time or not.
While little is known about Lonnie's skills and talents, it can be implied that she does have some skills, as seen in "Hidden Talent" when Bonnie mentions how she and Connie had won the school talent competition multiple times in a row.
Lonnie has only appeared in one episode in person, "Bonding". She appeared with Connie when Kim was around Bonnie's house due to being stuck to her by one of Professor Dementor's inventions. Before that both Lonnie and Connie were mentioned by Bonnie in "Hidden Talent". Bonnie mentioned how her sisters had won the school talent show 4 years in a row while signing up for the talent show herself, not intending to break the Rockwaller family tradition. She was not seen or mentioned anywhere else in the series.