Charles 'Chachi' Arcola is Fonzie's cousin, and it was through The Fonz that in season 5 Chachi and Joanie met. Chachi immediately had a crush on Joanie, and was always chasing her, although at first she couldn't stand him, as the seasons passed she came to like him more and more.
They started dating in season 7 (The episode is called "Hot Stuff"), when Chachi asks her if she'll date him, and she says "Maybe". Though they didn't go on their first date until the episode "Fools Rush In".
In Season 10 Chachi and Joanie moved to Chicago to pursue a singing career (The actors got their own spinoff called "Joanie Loves Chachi", It was cancelled after 2 seasons). When they came back to visit at the end of season 10 Joanie decided that she wanted to stay in Milwaukee and go to college, which put a strain on their relationship. Due to pretty much non stop fighting and Chachi's immaturity, in season 11 Joanie decided to break up with Chachi. In the series finale Chachi and Joanie reunite. Chachi proposes to her and in the second part of the finale they get married.