In Krypto Fighters, all Fighters who fight in the Adventure Mode (PvE) won't recover health automatically if they are injured. You need to recover their HP in the Gym's infirmary or use a healing item.
To recover your Fighters' HP, you can put your Fighters in the Gym's Infirmary. It is free but takes up some time.
Automatic full Healing and Revive from "Knockout" every 00:00 UTC everyday
Special for Genesis Fighter can full Healing and Revive from "Knockout" 1 times for free everyday
Use a healing item to instantly heal your Fighter. The following healing items can be bought from the in-game store: