All over the game you should see these green and grey circles that have the yin and yang symbols on them, these boost your health meter and ink meter. You'll mostly get these by beating bosses but you can also get them by destroying structures such as the wooden blockades, archer towers, enemy spawn generators, mini-bosses and even a few will be lying around in some stages. These should not be confused with the green and grey items that look like pots and vases as they aren't circles, these will restore your health and ink but will not increase it. You should get this towards the end of your fifth/final play through. I know it doesn't say it in the description, but you obviously have to do this in a single save slot so go read Every Legend Told for info on the single save slot.