Characters that don't come back in one piece

The narrative structure of One Piece ensures that many characters are put into scenarios where they can come back into the story, even after a long absence. That being said, even with the opportunities this structure provides, there are a handful of characters that we probably won't see more of due to various circumstances. The exact reasons behind each character vary, with most of them only united by the fact that the story seems to have, for better or worse, left them behind without an easy way to return. These characters have probably already made their last, if not one of their last appearances in the series.

7 Nero

Probably dead

Nero was a member of Cipher Pol 9 (CP9) who only knew four of the six Rokushiki techniques, which were not learned in Tekkai and Shigan. Despite being a cut below most of his counterparts, he was able to hold off Franky for a decent amount of time. Nevertheless, his own relative inexperience led to his expulsion after repeated failures. He was wounded by Rob Lucci and thrown into the ocean during a storm known as Aqua Laguna.

There is still no official confirmation that Nero was killed, leaving open the small possibility that he could have somehow survived the storm. But even if this was the case, it still wouldn't translate into further appearances. This is because Nero is fundamentally too unimportant of a character to definitively declare dead. Perhaps he can return at the end of the series, along with other characters who are not killed off, but given his immoral nature, likely death and relative insignificance, there is little that can properly bring him back into the series.

6 Hody Jones

Aged quickly

Hody Jones is one of the first serious villains of the post-Timeskip era One Piece. He is the captain of the new Fish-Man Pirates, and a blind follower of Arlong's ideals, knowing nothing of the people he claims to hate so deeply. He attempts to take over Fish-Man Island by force, which he would use as a springboard to enact an obscure plan to take over the world. But for all his talk, he and his crew proved incapable of taking on even the top-ranked soldiers on Fish-Man Island without some form of support.

To this end, they constantly consumed energy steroids, which would temporarily improve their strength physically. Although this led to premature aging, it gave them additional strength. But these steroids not only prevented their defeat, but eventually aged them beyond the point of fighting. Hody in particular took the most steroids and seemed to suffer even more than his peers. They were all in prison for their crimes.

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Given the fact of their rapid aging and the larger story's reduced focus on Fish-Man supremacist groups, it seems to make sense for Hody and his crew to return properly.

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3 Enel

Man on the moon

For all the significant lore revealed during the Skypiea arc, one cannot ignore that it takes place in a completely different terrain than the established blue seas seen for most of the story. This fact can be seen through characters with little investment in the events below, one of which is Enel. This self-proclaimed “God” seems largely content with his takeover, only frustrated by his inability to discover the Fairy Vearth, a legend told by the Birkan people.

After his loss to Luffy and more importantly, his loss of power in Skypiea, he instead refocused his efforts on finding the Fairy Vearth. It turned out that this was in fact the moon. Enel eventually encountered automatons on the moon, who were there to avenge their creator, who died due to an explosion caused by aliens on the moon. Outraged that aliens were desecrating the Fairy Vearth, he beat them out himself. After using his electrical powers to revive the defeated automatons, he discovers an army of them underground, as well as evidence linking the sky-dwelling races, and decides to rule over the moon.

Considering his seeming satisfaction with this goal and the apparent lack of people adversely affected by it, it seems unlikely that Enel will return, unless his moon base somehow enters the picture.

2 Jozu

In hiding

Jozu was a prominent member of the Whitebeard Pirates, and was one of its 16 division commanders (notably leading the 3rd division.) His Devil Fruit allowed him to transform his body into a nearly indestructible diamond, at the expense of mobility. His proper placement can prevent even a powerful slash from Dracule Mihawk, the world's strongest swordsman, from doing any damage.

However, the deaths of both his captain, Whitebeard, and crewmate, Ace, during the Summit War seemed to spell the end for the Whitebeard Pirates. This was expanded during the Payback War, where the remnants of the crew lost to the Blackbeard Pirates. Since then they have gone into hiding and the whereabouts of most of the members are currently unknown.

It's not impossible that Jozu could return, especially if what ever happened to the other surviving crew members becomes something more focused on, but there doesn't seem to be any possibility of that right now. At the moment, it can be assumed that he is in hiding, although where exactly is unclear.

1 Gaimon

Guarding the island with rare animals

Gaimon is a man who fell into a treasure chest and is claimed to be One Piece writer, Eiichiro Oda's favorite character. This fact, along with his hair that had grown out since then, had given him an odd yet endearing appearance. He fascinated Luffy enough to the point that the captain asked him to join the Straw Hat Pirates. Gaimon refuses, having grown attached to the rare animals on the island and feeling an obligation to protect them.

He has since developed a relationship with fellow future emperor, Buggy, when he himself was briefly on the island. A seemingly more permanent resident is Sarfunkel, a woman trapped in a barrel in a similar way to his own situation. The two seem romantically involved, due to her lovingly feeding him. The pair were two of many seen listening to Vegapunk's message, but this could possibly be their last appearance. Factors limiting Gaimon's involvement besides his desire to stay on the island include his limited relevance and low combat ability.