Mato Mato no Mi

The Mato Mato no Mi is a Paramecia-type Devil Fruit that makes the user capable of aiming at which ever "target" he decides upon from any location and at any time with anything at ease. "Mato" (的) means mark or target in Japanese. It was eaten by Vander Decken IX.

Strengths and Weaknesses
This fruit makes the user capable of aiming at whoever they touch with their hands. After his hand is imprinted with the memory of the target, the user is able to target the person from any location and at any time with anything at ease, regardless whether or not they are aware of the target's actual location. Even if the targets move away from their current locations, the thrown objects will keep aiming at them, immediately changing their trajectory in order to chase targets around. It seems that once the user designates a specific target by marking them through physical touch, the items they throw would then hurl towards the same location from where the user established the contact with the recipient, as seen when Decken used his power on Hachi by lightly touching his fellow Fishman around his back, and then hurtling a knife that swerved and then struck Hachi on that exact spot afterwards.

With the distance and acceleration of the projectile the user is sending it is a very dangerous ability for it can actually give a greater force in its attack as shown by Decken when he sent an axe towards princess Shirahoshi, where its acceleration gives a greater force to the axe blow, a blow that could actually be fatal. Apparently, the longer the distance between Decken and his target the greater the resulting momentum of the projectile thrown. And in case the target the user has chosen happens to be anywhere near them, the thrown projectile will actually even swerve back in a manner not unlike to a boomerang coming back to its thrower. Even a person as powerful as Luffy stated that the force behind the axe Decken threw was rather high. Furthermore while the user is affected by the sea water, the objects used to target the recipient(s) can travel underwater, without the sea affecting or decreasing the intended function. This ability works on both living beings and inanimate objects.

While this ability is capable of sending objects to the target, it doesn't give the user the capability of controlling the direction or trajectory of the item, it will just follow a path to the target, moving past every thing along the way. That would mean that if the item is blocked by something or somebody, the effect of the fruit will be nullified, and the item reverts back to being stationary. It will not act completely like a homing device. And as admitted by Decken himself, there are ways to protect one's self from his power. This is shown when the axe Decken threw cut his ship's mast and was blocked by Luffy, but it doesn't follow its target, in this case Princess Shirahoshi. The Mato Mato no Mi user can also memorize as many targets as how many hands he has (in Decken's case, two). However, in order to keep being able to target a certain person, the user must keep the hand that touched them from physically touching anyone else (it is implied it can't even be washed). Reasonably, owing to the fact that Decken must "mark" the target by touching them with his hands, he can be heavily disadvantaged in long-range combat against someone whom he had never marked before. Outside of combat, the power can serve as a sort of homing device to find the target's lcoation. Vander Decken IX used this idea to find Shirahoshi when he realized she wasn't in the tower. He threw a large piece of coral and then jumped on it, riding it to Shirahoshi's location. Other than that, the user is still affected by the standard Devil Fruit weaknesses as shown when Decken has to coat his ship and leave his ship in a Flutter Kick Coating, despite being a fishman.

Usage
Decken has been using this fruit's power to launch projectiles at Shirahoshi from great distances, in order to threaten her for marriage, even aiming to take her life. This was the reason why the princess is locked up in the Hard-Shell Tower for a decade, to block off all incoming projectiles. Due to Decken keeping his right hand memorized with Shirahoshi's touch, he can only use his left hand to memorize other targets. Shirahoshi, not knowing about Devil Fruits, referred Decken's power to be a "curse" named Mato Mato.

Trivia

 * This Devil Fruit marks the first time a Devil Fruit was consumed by a fishman in the current storyline.
 * Like Bartholomew Kuma, Decken has to wear gloves to control some of the effects of his Devil Fruit. Also, like Kuma's Nikyu Nikyu no Mi, the hands of the user play a crucial role in implementing this fruit's power. It even follows the theme where the user can send objects and people away.
 * This Devil Fruit is the second occasion in which it was mistaken to be a curse, the first being the three fruits consumed by the Gorgon Sisters.
 * Similar to Mr. 2 Bon Kurei's Mane Mane no Mi, this fruit also follows a "memorizing" theme that the user must initiate with the use of their hands.
 * This Devil Fruit might have been called a curse due to the fact that the current user is a fishman and it has robbed him of his ability to swim.

Links

 * Track & Trace - A Wikipedia article on the device that is similar to Decken's ability.