Phosphor

Phosphor, full name Flashfire Fist: Phosphor is an Ultimate Move performed by Shoto Todoroki and Dabi using the Half-Cold Half-Hot and Blueflame Quirks, respectively.

Description
Phosphor allows Shoto to channel both halves of his Quirk throughout his body via his circulatory system, using his heart as a generator to circulate the chilled and hot blood around his entire body, merging the two halves of his Quirk into a single ability. The move grants Shoto an incredible defensive and offensive advantage against most opponents, especially those wielding pyrokinetic Quirks. The flames created by the technique are described as being a “Cold Fire” of sorts, allowing him to both burn and freeze his opponents.

This move is first developed during a training session between members of Class 1-A in a U.A. courtyard. It is described by Shoto as his body coming together "as one," as well as creating "a body even Dabi can't burn."

This move gets its name when it's used against Dabi during the second war. Shoto uses it to protect himself from Dabi's Jet Burn, as well as using it to power his Ultimate Moves, Coldflame's Pale Blade and Great Glacial Aegir.

The move takes a certain amount of concentration and focus to build up, so if he loses control, he has to build it back up again.

After being seemingly defeated by Shoto using this technique, Dabi is able to replicate it using his own Quirk to reinvigorate his body and cancel out Shoto's ice.

Trivia

 * Upon first utilizing Phosphor, Shoto’s body resembles his father’s second hero costume.
 * Phosphorus, which Phosphor is named after, is a chemical element that exists in two major categories: white phosphorous and red phosphorous. White phosphorous emits a faint glow when exposed to oxygen at a cold or room temperature. Red phosphorous is created due to a chemical reaction that occurs when white phosphorous is heated to 250 degrees Fahrenheit (121 degrees Celsius). Therefore, the Ultimate Move’s ability to combine both the fire (signified by Shoto’s red hair side) and ice (Shoto’s white side) is similar to the chemical element.