you would benefit from reading this before trying to understand this but i'll try to give a explanation
in my theory a devil fruit is created from a persons thoughts and desires. in order to get the fruit the very first person who had the thought or desire died and the ocean took them and turn them into the fruit and since a island sits in the water it just has the tiny seed go up from the water into the land and grow as a random fruit.
In my theory Nika was a person with a desire to "make people happy and save slaves" and when he died the ocean turned him into a fruit.
some guy came along and ate it and did what the fruit wanted and they called him "joy boy" and they started the legend about Nika that he wasn't a person.
all zoan fruits are sentient, all this one wants is for it's vessel to "make people happy and free them." and Luffy keeps doing that unknowingly pleasing the fruit but when he fails right upon achieving the goal the fruit takes matters into its own hands and rebuilds Luffy's heart taking 90% of his body cause that's a fair exchange. You get to keep living i get more then half of your body.
what does it mean that the fruit has 90% of Luffy's body? that means it's playing Luffy. Luffy's current personality is simply trying to keep breathing and not much else but the fruit has been watching Luffy through his eyes, his reflection. and it decided that it's not fair to have what it desires taken from it so it rebuilds Luffy's heart to get what it wants.
its not evil it just wants what it wants.
Now Luffy doesn't know his fruit ends or where he begins, he's aware that his body wrong and something isnt right but he doesn't know how to describe it
thank you for coming to my ted talk.
