Frost

Frost is the Quirk used by Rei Todoroki and her children, Fuyumi and Natsuo.

Description
Frost gives the user the capability of manipulating and producing ice around their body.

Rei's possession of this Quirk was what directly resulted in her Quirk Marriage with Enji Todoroki to create a child that possessed a perfect fusion of it and his Hellflame Quirk. The second and third children, Fuyumi and Natsuo, inherited the Frost Quirk, while the eldest, Toya, inherited a stronger version of Hellflame, but Frost's resistance to ice and weakness to fire. Toya later manifested ice as part of his Quirk after coming close to death. The youngest son, Shoto eventually developed the combination Quirk that Enji so desired, Half-Cold Half-Hot, containing the strength of both Quirks as well as the way to alleviate their respective weaknesses.

Weakness
Rei cannot use her Quirk for extended periods of time as overusing it or prolonged usages can cause frostbites to develop throughout her body. Her body temperature drops drastically if she surpasses her limits as it may also cause hypothermia.

Usage
Rei is first seen using this Quirk to attempt to heal Shoto after she instinctively throws boiling water at him.

Rei is later seen able to coat her body in her Quirk in an effort to protect herself from the blazing inferno caused by Endeavor and Dabi during their fight, stopping them from blasting off into the air. Fuyumi and Natsuo soon do the same, using their Quirks to protect themselves from the flames while trying to cool their brother down.

Frost ended up being vital in stopping Dabi's self-destruction. While the three of them sustained major burns in the process, Rei, Fuyumi, and Natsuo were able to use their ice to temporarily route Dabi's explosion, giving time for Shoto to arrive and use his Phosphor-empowered Great Glacial Aegir to freeze over Dabi's inferno and defeat the villain for good. Shoto admitted that without his mother and siblings' help, his power alone wouldn't have been enough to stop Dabi.

Trivia

 * Frost's name was never given in the manga. It is only named as such in the anime.
 * Frost is said to give the user a sensitivity to warmer weather, as well as a liking for cooler weather.