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. This transfer of heat can even happen without physical contact, as demonstrated when Oven seemingly projected heat energy to Pekoms' gun to set it ablaze. However, the power is ineffective against opponents who possess a resistance to heat. Other than that, the user is also affected by 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 or transferring heat to his own clothing and 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,Nekkai Jigoku?, meaning "Hot Sea Hell"): Oven heats up his hands and submerges them in the ocean, causing the water in an enormous radius around him to boil. The water will grow hot enough to cause significant damage to any aquatic creatures or ships within it, making it particularly powerful when fighting against fish-men. 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).
Nekkai Gyorai (熱海万来,Nekkai Gyorai?, meaning "Hot Sea Torpedo"): A variation of Nekkai Jigoku. Oven heats up his hands and submerges them in the ocean, but unlike Nekkai Jigoku, he does not make the whole ocean boil, only portions of it, creating several underwater heat beams that travel like torpedos towards his opponent. It was first used against Wadatsumi when he tried to escape from the Big Mom Pirates with the Thousand Sunny. This is called Tropical Torpedoes in the Viz Manga. The kanji of this attack can be read as Atami Banrai, which literally means "Atami's many guests".
Heat Denasshi (熱風拳,Hīto Denasshi?, meaning "Hot Wind Fist"): Oven heats up his forearm and throws a punch of tremendous power. He first used this against Pekoms.Hitodenashi (人で無し,Hitodenashi?) literally means "you brute", meaning the attack name can also be read as a statement against Pekoms. This is called Heat Beating in the Viz Manga.
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:
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 whirlwinds, 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.