Arena inside has a sort of black market NPC at night time where you can pay some height for a hp booster I believe it's called. At the end of day every day it will permenantly boost your hp by 5. If that isn't enough for you then there's succubus decks which have buffs that steal hp from the enemy and whatever hp they steal through those buffs is granted to you as permenant max hp allowing you to gain far more max hp than the hp booster could ever hope to give you. Thanks to succubus decks my character's max hp growth has outpaced all the enemies.