The Netsu Netsu no Mi is a Paramecia-type Devil Fruit that allows the user to emanate heat from their body, making the user a High Heat Human (高熱人間,Kōnetsu Ningen?, Viz: "Bakeman"). It was eaten by Charlotte Oven.
The user is capable of heating parts of their body to temperatures greater than most people can withstand. This greatly aids them in a confrontation, as Oven showed in his clash with Pedro. They can also transfer the heat to other objects, most notably the opponents' weapons, making them too hot to wield. However, the power has no effects against opponents who possess a resistance to heat. Other than that, it is weak from the standard Devil Fruit weaknesses.
Oven uses his fruit in combat, transferring heat to other people's weapons in order to make them too hot to hold. Oven can also transfer his heat to his own weapons to heat them up and even set them alight. He can also grab onto an opponent and directly burn them.
Nekkai Jigoku (熱海温泉（ねっかいじごく）,Nekkai Jigoku?, meaning "Hot Sea Hell"): Oven heats up his hands and submerges them in the ocean, causing the water in a large radius around him to boil. The water will grow hot enough to cook fish within it, as well as cause substantial damage to any ships sailing on it. It was first used to stop the Nostra Castello from escaping. This is called Tropical Torment in the Viz Manga. The kanji of this attack can be read as Atami Onsen (Atami is a city in Japan known for its hot springs, or onsen).
The Netsu Netsu no Mi's powers to exude high amounts of bodily temperature is very similar to those of the non-canonAtsu Atsu no Mi and Kachi Kachi no Mi. However, there are subtle differences between these three fruits' powers:
The most noticeable difference is that the Netsu Netsu no Mi can consciously control the amount of heat being transferred into objects: while heat is transferred into any object through the laws of thermodynamics, the Netsu Netsu no Mi powers appears to grant complete control over whether to do so and by how much, superseding the second law of thermodynamics. Neither the Atsu Atsu no Mi nor the Kachi Kachi no Mi has shown the ability to bypass this law.
Unlike the Atsu Atsu no Mi, the Netsu Netsu no Mi has yet to show the ability to regulate one's temperature by degrees, nor produce visible energy that can be thrown as projectiles. Also, the Atsu Atsu no Mi powers have been able to manipulate external heat such as that of lava and hot air for tidal waves and whirlwind, which the Netsu Netsu no Mi has not demonstrated.
Similar to the Kachi Kachi no Mi, the heated up body parts provide a level of enhanced physical defense against blades and blunt weapons, but unlike the Kachi Kachi no Mi, this ability does not physically harden one's body parts.