Joanna "Jo" Marie Polinaczek was played by Nancy McKeon. Jo was first introduced in 1980, arriving at the Eastland Academy on her motorcycle. Her age throughout the series was roughly 15 to 23. She almost immediately formed a dislike for Blair Warner; Jo found Blair stuck-up, and Blair found Jo classless and manly. In a memorable introduction, Blair introduced herself to Jo, to which Jo replied, "Charmed; Gloria Vanderbuilt." Jo is Polish American.
Jo convinced the gang to steal the school van and use fake IDs to buy drinks at a bar. The van was wrecked and the girls were forced to work in the Eastland cafeteria to make up the repair costs. The girls were also placed on house probation and were forced to live in a room adjoining Mrs. Garrett's for a year; when the punishment expired, all four girls found other living arrangements, but were responsible for cleaning and painting their former room. While painting, their friendship began to rekindle, culminating into an all-out paint fight, which severely damaged the hardwood floor, requiring expensive repairs. The girls, once again close friends, decided to move back in together and continued to work in the kitchen to pay off the bill for the damaged floor.
Blair's attitude toward Jo worsened when Blair's boyfriend asked Jo to a country club dance. However, Blair stood up for Jo when her would-be love interest tried to assault and humiliate her on the ninth green. When asked why Blair stood up for Jo, Blair stated, "At least when I insult her, I know what I'm talking about!" Another problem for Jo came when she shoplifted a blouse for Mrs. Garrett's birthday; Mrs. Garrett was subsequently arrested when she went to exchange the blouse for the smaller size, not knowing the garment was stolen.Early on, Jo's sailor boyfriend, Eddie Brennan, came to Eastland and convinced her to marry him. They planned to elope in West Virginia, where the marriage age was lower, but Mrs. Garrett and Blair tracked her down at a nearby motel to stop her. Jo had by that point already begun to change her mind when she realized just how difficult teenage marriage would be. Jo returned to Peekskill without Eddie. Eddie visited again the following season, but things had changed between them. They were moving in different directions, and keeping the long distance relationship going was proving to be very difficult, so they decided to see other people for the time being. Eddie returned late in the fifth season, now an officer in the Marines, but Tootie discovers he was married to a girl in Italy.
Later, Jo developed a friendship with her 26 year old English teacher, Miss Gail Gallagher, who was raised in a similar environment to that of Jo. Gail inspired Jo to think about becoming a teacher herself and Jo went as far as to calling her by her first name. When Jo learned from Blair that Gail was quitting Eastland, she became very disappointed, believing that the reason was for a higher-paying job. She was then crushed when she found out from Mrs. Garrett that it was because of something unthinkable – terminal illness. Jo was so shocked that she stopped going to Gail's class and shunned her completely. She tried to keep herself busy with things that would distract her from thoughts of Gail and went as far as breaking her evening curfew. When Mrs. Garrett and Tootie confronted her about her strange behavior, she managed to overcome the shock and fear and come to terms with Gail. Jo associated the fact that Gail's dying with her friend Gloria's death.
Many of Jo's stories revolved around her tomboyish ways. Jo got into verbal spars with girls who didn't think she was "feminine enough," and boys (including some of her boyfriends) were threatened by her mechanical aptitude, which she displayed by getting a job at a local garage.
In the later seasons, Jo's tomboyish image softened considerably, and was rarely raised as an issue again. Her relationship with Blair became more friendly as time went on (Jo would even refer to Blair as her best friend by the last season), although the two still argued, traded wisecracks and made fun of each other from time to time. Jo graduated from Eastland as valedictorian of her class, and attended Langley College with Blair, graduating with a degree in education. In one flashback-heavy episode, Jo relayed her latest ongoing spat with Blair to a computer, which analyzed the data and concluded that Jo should stop being friends with Blair. At the end of the episode, Jo quietly threw the computer's printout away. In the last season, Jo married Rick Bonner, a concert musician, and she asked Blair to be her maid of honor.
During the show's TV film, it's revealed that Jo has become a police officer and is unable to attend the reunion with her friends because she has to escort a witness to testify in a major trial. She and Rick also have a teenage daughter named Jamie who is shown to be very similar to her mother.