JoJo

"Jotaro Kujo... apparently, his friends at school took the "Jo" from Kujo and the "Jo" from Jotaro to nickname him "JoJo"... What a riot !"

- Japanese Police Officer,

JoJo (ジョジョ) is a nickname shared by each of the main protagonists of JoJo's Bizarre Adventure, derived from the recurrence of "Jo" in their names.

In the text of the manga during Phantom Blood and Battle Tendency, it is used most frequently in reference to Jonathan and Joseph Joestar.

Each of the protagonists is also descended from the Joestar Family, and as such all bear the Joestar Birthmark.

Non-Protagonists
The following characters are not protagonists in the series, and some do not even belong to the Joestar bloodline. However, it's notable that their names are able to be shortened as "JoJo":

Origin
The name "JoJo" was derived at a where Araki and his first editor, Ryosuke Kabashima, had their first meeting pertaining to the series. The two had decided on "Jonathan" as the series' protagonist, however, Araki also wanted the initials to match in a way similar to the American filmmaker, (S.S). They eventually landed on the name, "Jonathan Joestar" (J.J) and thus the nickname "JoJo" was born.

Jonathan was rumored to be named after "Jonathan's Family Restaurant," however, both Araki and Kabashima confirm this was an urban myth that formed among fans back in the day. Araki had also brought up the name during a 2006 interview, but he claims the interviewer had mentioned it and he just went along with it by accident. Their later meetings during Part 2 and Part 3, however, were ironically at said Jonathan's restaurant.

Trivia

 * Giorno Giovanna and Josuke Higashikata are the only main protagonists who were never referred to by the nickname "JoJo" (or the stylized "GioGio") by other characters in canon works.
 * The only time the stylized "GioGio" is mentioned by another character is during the ending of the light novel Purple Haze Feedback wherein Pannacotta Fugo and Giorno Giovanna discuss the events of the story, Giorno tells Fugo to "do him a favor" and call him "GioGio" instead of "Boss", Fugo replies by swearing his allegiance and refers to him as "GioGio".