Not technically “World of” series but let’s talk about it. Baroque Works
Baroque Works is an underground organization headed by Pirate and Shichibukai Sir Crocodile.
Baroque Works has a hierarchical system. All male agents are paired with a female agent, except for Mr.2 because he’s both???
The male agents are always “Mr. #”. Lower the number, higher the rank, and Crocodile is Mr.0. The female agents are named after days, with greater holidays having higher rank. Nico Robin AKA Ms. Allsunday because Sundays are always a day of break. She is also Crocodile’s partner.
Notable members include Crocodile obviously, Nico Robin (Ms. Allsunday) as she becomes a member of the Straw Hats, Daz Bones (Mr.1) has he appears again in Impel Down, and Bon Clay (Mr.2) as he becomes close ally to the Straw Hats and saves Luffy multiple times.
I hope you enjoyed this!