Mera Mera no Mi

The Mera Mera no Mi is a Logia type Devil Fruit that allows to user to transform into fire at will as well as control it. "Meramera" is the noise a fire makes when it burns. It is called the Flame-Flame Fruit in the Viz Manga and FUNimation dub, and the Flare-Flare Fruit in the 4Kids dub. It was eaten by Portgas D. Ace. Its powers were first hinted at when Ace visited a town in Drum and apparently caused the snow to temporarily stop from falling there for a day.

Strengths and Weaknesses
The fruit's major strength like other Logia types allows the user to produce as well become the element they control. In this case, Ace is able to create, control, and become fire. With this in mind, Ace is able to create fire-based attacks. He is also able to avoid normal attacks by turning into fire and allowing the attack to pass through him. Turning into fire also has the added effect of burning an opponent depending should the attack be closed range and such.

The fruit so far doesn't seem to have any specific weaknesses outside the standard Devil Fruit weaknesses.

Usage
The fruit's bestowed powers, as used by Ace, have been used for a variety of ways. Ace notably uses them usually either to fight or to propel his raft. Ace shares Luffy's penchant for naming his techniques, with the only diffrence is, while Luffy adds the phrase "Gomu Gomu No", followed by the specific name of the attack (usually inspired by a certain object or concept), Ace names his attacks in japanese and often constructed in two-word phrases and based on mythology. During Ace's cover story, the powers of the fruit allowed Ace to rescue a Marine officer and some documents on board a burning ship. Despite being weakened by the Yami Yami no Mi severely during his fight against Blackbeard, Ace was still able to skillfully use the fruit's power to inflict some damage to his former subordinate. The named techniques that are used by Ace that involve the Devil Fruit are as follows:
 * Kagerō (陽炎, Heat Haze): Ace shoots a stream of fire from the palm of his hand. This was first seen being used to stop Smoker from catching Luffy. This is called Sun Flare in the Viz Manga and Heat Haze in the FUNimation dub.
 * Hiken (火拳, Fire Fist): Ace's apparent signature attack and the source of his nickname "Fire Fist Ace". Ace turns his fist into flames and launches it as a column of fire at his target. This attack is so incredibly powerful that it is able to plow through several ships in one shot. This is called Fire Fist in the Viz Manga and FUNimation dub and Heat Fist in the 4Kids dub.
 * Higan (火銃, Fire Gun): Pointing the index and middle fingers of both hands at an opponent like guns, Ace turns his fingertips into flames and shoots bullets made of fire at them. This technique was first seen when Ace retaliated against Van Auger's attack.
 * Enkai (炎戒, Flame Commandment): Ace amasses a large amount of flames around his body (usually by first swinging his outstretched hand around him, summoning the flames) in preparation for an attack. This is apparently the prerequisite technique before Ace can use Hibashira.
 * Hibashira (火柱, Fire Pillar): Used following Enkai. Ace shoots the collected flames straight upward, destroying a target above him. This technique and its prerequisite technique were first seen when Jesus Burgess threw a hotel at Ace.
 * Hotarubi (蛍火 火達磨, Firefly Fire): Ace creates many small, glowing greenish fireballs that float around an enemy. The idea behind this attack's name is that before he attacks with them, the small fireballs look like fireflies in the air. This move is apparently used in preparation for Hidaruma and was first seen in Ace's battle against Blackbeard
 * Hidaruma (蛍火 火達磨, Fiery Doll): After using Hotarubi to send the fireballs floating around in proximity with the enemy, Ace sends them all flying into the target(s) all at once, burning them. This was first seen being used in Ace's battle against Blackbeard after Blackbeard performed his initial attacks.
 * Shinka: Shiranui (神火 不知火, Sacred Fire: Mysterious Light at Sea): Ace launches two long lances made of fire at his opponent's chest. The lances have the combined effect of simultaneously burning an opponent while piercing their chest. This was first seen being used against Blackbeard when Ace was being dragged by Blackbeard's power.
 * Jūjika (十字火, Cross-fire): Ace puts his index fingers together in a cross shape before launching a cross-shaped column of flame directly at his opponent. In the anime, after crossing his fingers, a cross-shaped ray of light can be seen emanating from the fingers at the target followed by the column of fire. This was first seen being used against Blackbeard after Ace attacked Blackbeard with Shinka: Shiranui. This attack's name is a pun since "Juujika" refers to crossfire from weapons, yet Ace is making an actual cross out of fire.
 * Daienkai (炎戒, Great Flame Commandment): A stronger version of Enkai where Ace summons great spiralling flames centered on his location before ammassing it at a focused point (i.e. the palm of his hand) for a technique like Entei. In Unlimited Cruise, Ace activates this move by first bending his knees slightly and then placing the outstretched palm of his hand towards the ground (with his other hand gripping the wrist for support), where the flames manifest
 * Entei (大炎戒 炎帝, Flame Emperor): Ace's ultimate technique. Ace creates a gigantic fireball resembling the sun and hurls it at his opponent to try to obliterate them. This was first seen being used during the final moments of Ace and Blackbeard's battle. "Entei" is a homonym with "Enkei" meaning "circle", a reference to the attack's shape. Additionally, Enti was the Inca god of the sun.
 * Enjōmō (炎上網, Blazing Net): Ace creates a circle of fire around a selected area, preventing anyone from entering or leaving it. This was first seen being used in a flashback against the Marine corps in Arabasta under command of Smoker, and was named in a flashback against Whitebeard.