Every hero has a Health attribute, which you can see on the hero screen.
This increases when you
- Level up the hero (using crystals and coins) / promote the hero (using Hero medals and coins)
- Star up the hero (using Hero shards)
- Get Max Health permanent upgrades
- Get hero-specific health bonuses on promotions (e.g. Mr. Autofire gets +50 Health at Level 10, Miasma gets +5% at :Level 30)
- Get Health skills (using Rings and coins) - some are fixed amounts and some are percentages
- Get all-hero bonuses by promoting other heroes (e.g. Fursus, Frostina and Arctor at Level 20 gives +200, +5% and 6% respectively)
You can estimate the Health (and Attack) using the Hero Stat Simulator