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Theory: Devil Fruits have individual wills and personalities which affect their owners.

The Devil Made Me Do It In Enies Lobby, as Kaku and Kalifa are about to receive their devil fruits, Jyabura explains that devil fruits work because they contain demons; Lucci tells him he’s full of it, the viewer laughs, and we come away with an understanding that people cannot eat two devil fruits, but no reason why. What if Jyabura wasn’t all wrong? A number of small details come together to act as evidence that devil fruits have personalities and wills which subtly (or sometimes not subtly) affect the personalities of those who eat them.
This is a difficult theory to find evidence for, for two reasons: first, because there are few instances where we see characters both before and after they eat devil fruits; second, because Oda’s storytelling style often gives characters powers which match their personality anyway (for example, is Van Der Decken a creepy stalker who has a thematically appropriate fruit, or does the fruit make him a creepy stalker? Does Corazon/Rocinante prefer not to talk because he has a silence fruit, or did Oda give a silence fruit because he doesn’t like to talk?

However, enough evidence exists that I believe this theory has a strong chance of being right. Moreover, the possible consequences are extremely far-reaching, an exciting trait in any theory, and enough to make this worth considering.
Ch-ch-ch-changes
We see, through the story, only a handful of characters before and after eating their fruits. Most of those characters we first see as children, before their personalities are fully developed, or hiding their true personalities. Here is a list of those characters: (EDIT: I’m aware I missed several. My bad.)
* Luffy (half a chapter as a kid) * Buggy (a very brief flashback) * Kaku and Kalifa (pretending to be Galley-La workers, followed by stealthily attempting to assassinate Icerberg) * Blackbeard (one short flashback, pretending to be Whitebeard’s family, before he ate the Darkness fruit; two short scenes and a few dialogues after eating the Quake fruit)
* Ace (one arc as a kid) *Sabo (one arc as a kid, followed by a few chapters as “Lucy”
* Momonosuke (one short flashback)
* It’s possible that there are other characters we see before they eat their fruits, but it is unknown (we don’t know for certain whether, say, Whitebeard had his fruit in his various flashbacks, or when Doflamingo ate his).

Of these, the most interesting are the CP9 agents, Kaku and Kalifa. Kalifa’s personality is extremely hard to read while she is Iceberg’s secretary. However, from that section, her break-in in the Galley La office, and her brief actions on the Sea Train, it’s clear she’s extremely decisive and powerful, quite willing to get her hands dirty, and talented with a unique thorn whip. However, after eating her devil fruit, her fights with Sanji and Nami are absurd and comical. She never wields her whip, she uses almost no Rokushiki moves, and she relies primarily on her gimmicky devil fruit rather than simply kicking them both to death. Her personality changes drastically after eating her devil fruit (and not for the better). She goes from deadly assassin to dumb blonde. Similarly, Kaku becomes far sillier, more focused on arguing with Jyabura than on defeating his foes, compared to the efficient assassin we saw in Water 7 (or the friendly parkourer he was pretending to be).

Of the others, it is difficult to draw conclusions from so little data; once we see more of Sabo, and post-timeskip Blackbeard, the effects of the Mera Mera and Gura Gura fruits may become more obvious, but there is simply too little pre-fruit from the others to draw effective comparisons (but see below for more on Ace, Blackbeard, and Momonosuke). However, looking at Luffy reveals something very interesting. In Chapter 1, he first tells Shanks “I want to be a pirate.” Just that, just a pirate. However, after eating the Gum Gum fruit (and after being rescued from the mountain bandits and various other important pieces of character development), he tells him, “I am going to become king of the pirates.” This phrase is later described as being “just like what Roger said”. There is heavy speculation on this forum about whether Roger had a devil fruit at all, as well as whether he had the Gum Gum fruit specifically, but if the theory of Devil Fruit Wills is correct, an inherited fruit may well explain the strange connection between these two.
You Are What You Eat
The description of Blackbeard’s Yami Yami fruit is unique: it is the “most evil” of the devil fruits. That’s a very odd description; abilities are morally neutral. When Blackbeard described his fruit as “evil”, it could only be with an awareness of something inherently evil within the fruit, perhaps a personality that had begun to overtake him. This is not to say that Blackbeard was ever a saint: he killed a nakama to get the fruit in the first place (albeit after some hesitation). However, it suggests that he has been made more evil by the fruit, perhaps against his will. Because of his god-killing D nature, this theory predicts that Blackbeard’s story will end with him overcoming his devil fruit and allowing Luffy to defeat him.
Inversely, everyone who has known Ace has been surprised that he had no interest in leading. After setting out on his own and reluctantly joining Whitebeard, he eventually accepts Whitebeard as his father and has no interest in becoming the pirate king himself, even as Whitebeard was (according to the marines) grooming him to take on the mantle. Everyone seems surprised that with all his power, he refuses to lead. However, Ace’s childhood dream was never to become pirate king, it was to get out from under his father’s shadow. Having achieved that, and having found a new father figure, Ace is able to resist the destructive power of the Mera Mera fruit (which he used for years before joining Whitebeard, gaining a reputation for razing villages and sinking battleships) with his own willpower (and perhaps his will of the D). It remains to be seen how Sabo will respond to him.
Natural And Artificial Flavoring
Thus far, I have done a great deal of speculation and theorizing based on character personalities which could honestly be explained by normal character development; no one has wondered why Luffy is ambitious or why Blackbeard is evil, it fits well within their characters. However, this setup was necessary to compare with an area that the series has only begun to explore: Vegapunk’s experiments on devil fruits.
There are exactly three cases where we know for a fact that eating a devil fruit imparted a personality: Mr. 4’s Dog Gun, Spandam’s Elephant Sword Funk Freed, and Smiley the adorably terrifying poison salamander. Before eating devil fruits, they were inanimate objects; after eating devil fruits, they gained autonomy and personality. These are very minor characters, but the fact that their existence is even possible should make us suspect that devil fruits are more than just powers: there’s something inside them that changes how their eaters act. Even if it is merely the animal spirit of the Zoan, these fruits impart something on their eaters which has will.
In contrast, the Pacifistas, whose laser powers were derived from Kizaru’s devil fruit, have no will at all; they are simply automatons which follow orders and enact rote programming. Vegapunk can give objects sentience with devil fruits, but their devil fruit power not give them will because it is artificial. This is why Vegapunk’s efforts to create artificial devil fruits are considered “failures”, even though Momonosuke’s dragon fruit appears to be quite effective, and the results are good enough for Kaido to desire enough to feed an army: these devil fruits are incomplete because they have no will. They were created in a lab, so there is no will to inherit.
Will I Am
This brings up the most important consequence of this theory: its interaction with Inherited Will. Inherited will is a key concept in One Piece (and the basis for what is probably the most popular theory on this board), and this theory falls perfectly in line with it. A devil fruit user, in this theory, gains some of their willpower from the fruit that they eat, and perhaps in turn imparts their desires back into the fruit. When it reincarnates, the next user inherits that will. Luffy inherited Roger’s hat and middle initial, and if Roger wielded Gum Gum powers, Luffy also inherited his will through the fruit. For Sabo, it was essential that he eat the Mera Mera fruit to inherit Ace’s will. And Blackbeard, perhaps unknowingly, has inherited evil desires from whoever last held the Yami Yami.
Back to the Future
If this theory is correct, it raises many questions, and casts some older questions in a new light:
* Where do fruits originally come from? Where do the fruits gain their wills?
* How long have these wills been manifest? Is there a connection to the ancient civilization of the Poneglyphs?
* Is the will inherited through fruits connected to the will inherited through the D?
* How many people know about this? Vegapunk and the World Government very likely, perhaps also some Celestial Dragons?
If Doflamingo knows, is the will of the Ope Ope fruit essential to accessing the great treasure of Mariejois?
* What are the psychological effects of eating two fruits? How will the Gura Gura fruit affect Blackbeard’s will?
* What is the connection to the ocean? Does the sea hate the will of devil fruits, rather than their power?
tl;dr
One Piece is a very complex story about a very complex world. Oda is known to foreshadow key plot points decades in advance, and he has been thinking about devil fruits since the very beginning. I believe they are more than just magical power mcguffins: I believe devil fruits have their own personalities and wills which they impart on those who eat them.

*All rights go to CharlieLady

The official One Piece website also revealed the character model for “Corazon” Rocinante and the Op-Op Devil Fruit.

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lol