One Piece's Wano Country arc has taken a bit of a break in the manga as series creator Eiichiro Oda has briefly returned to the outside world to follow up on the Reverie mini-arc that abruptly ended after seeing Luffy enter Wano officially. As each new chapter of the series continues to make huge reveals, it's increasingly apparent that Oda is setting the stage for an even bigger conflict in Wano than originally expected. Chapter 957 sees the Navy prepare for their part in the action, and as they gather intel it's soon revealed just how powerful Luffy's grandfather really is.
Given how strong he's been in the series thus far, and his reputation as a hero, it might be surprising to see that he's still a Vice Admiral after all his years of work. But Chapter 957 of the series reveals that Garp doesn't want to be promoted to Admiral because then he'd have to work directly under the Celestial Dragons.
As Sengoku explains in the chapter, Garp originally made a name for himself as a naval hero for surprisingly allying himself with Gol D. Roger in order to take down the threat of the Rocks pirates. Sengoku reveals that Garp never wants to talk about this fight because not only did he work with pirates, he did so in order to protect the Celestial Dragons. This strongly opposes his moral code, but he's never been in danger of losing his position for insubordination for refusing to serve the Celestial Dragons.
Becoming an admiral means that he'd have to work under them directly, but his accomplishments as a hero keep him from being killed or losing his position in the Navy for openly rejecting their favor. Fans have seen this side of Garp before, like when he pretended to lose to Luffy at Marineford or was revealed to have taken Ace in after Gol D. Roger's death. And now we know why he can do these things without major ramifications.
Garp's stature allows him to be openly brazen, and it seems that he's fine as long as he's a weapon for the Navy. But that also begs the question of just how much of a threat the Rocks Pirates once were if Garp has essentially gotten a free pass for saving the Celestial Dragons long ago.
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has since been collected into over 80 volumes, and has been a critical and commercial success worldwide with many of the volumes breaking printing records in Japan. The manga has even set a Guinness World Record for the most copies published for the same comic book by a single author, and is the best-selling manga series worldwide with over 430 million copies sold. The series still ranked number one in manga sales in 2018, which surprised fans of major new entries.