You know what? The King probably did it.
Right now, Brook thinks that Shuri killed King Leuven. I think we all suspect there's more to this story. Shuri is a cute girl. Cute girls get redeemed 90% of the time in One Piece. It's just math. So, chances are, Shuri didn't REALLY kill her father.
So, what if King Leuven has the Op-Op Fruit? We know Brook was the captain of a battle convoy, so Esperia ends up at war with someone. Shuri could get wounded in some battle. And then Leuven uses the Op-Op Fruit to save his daughter's life with the Immortality Surgery. Brook comes along at just the wrong time, and he thinks Shuri killed Leuven, even though Leuven chose to sacrifice his life for Shuri. Maybe Leuven was keeping the fact that he had the Op-Op Fruit a secret? Maybe that's why Candelle's so protective of the King? That just feels like the kind of twist Oda would do.
Chances are Shuri will end up with amnesia after the attack that wounded her, because "Gunko" literally means "Army Child" in Japanese. Which feels like a hint that the God's Knights didn't actually know who Shuri was when they "adopted" her. Maybe the attack even blew part of her head off or something, destroying her memories that way, who knows? The Immortality Surgery could have repaired Shuri's body, but not necessarily her mind.
Either way, Imu "adopts" Shuri. Imu's probably looking for the Op-Op Fruit. Because it can grant immortality, so you wouldn't want it to fall into just anybody's hands. So, maybe Imu just "adopted" her because she's the only lead to where the Op-Op Fruit could have gone? Or maybe Imu or the God's Knights have a way to "sense" when someone else is immortal? If they have a way to "sense" when someone's immortal, they'd probably be VERY interested about why this random girl is immortal, and decide to take her with them. Or maybe it's as simple as Shuri's blue hair reminding Imu of Lili. Or maybe it's due to Shuri's special eyes? I don't know.
Now, there's been NO sign of Leuven having the Op-Op Fruit. But, it's one of the only ways to give someone true immortality in this world. So, there's a pretty good chance of it coming into play at some point in this story. Gunko's immortality doesn't seem to be linked to any sort of Contract Mark, since no one else at her level seems to be immortal like she is. Only the Gorosei seem to be as long lived, and as far as we know, Gunko doesn't have some special Gorosei-level Contract Mark But, we'll see. I guess Shuri's immortality could be the result of something we don't even know about, yet.
