As you make your way through Code Vein, you’ll rely on Regeneration to top up your health, just like using the Estus Flask in Dark Souls.
Initially you can use Regeneration just three times between resting at Mistles, and it doesn’t restore much health. Be diligent as you play through Code Vein, however, and you can unlock more uses and make each one more potent.
By thoroughly exploring each of Code Vein‘s areas and defeating certain enemies, you can find two items that improve your Regeneration capabilities. If you find a Regen Extension Factor it can be used to permanently increase the number of times you can use Regeneration between visiting Mistles, while if you find a Regen Activation Factor it can be used to increase the amount of health that each use of Regeneration restores.
Unfortunately, simply picking up a Regen Extension Factor or Regen Activation Factor doesn’t boost your Regeneration capabilities. You need to use them to gain their benefits. To do that, simply bring up the game’s menu, go into your inventory and then find them in there. You can then select and use them, increasing your Regeneration abilities.
Try to find as many Regen Extension Factors and Regen Activation factors as you can in Code Vein to maximise your survivability.