The more you level the more you get HP and if you decided to join a league check out there leuage shop they have different types of necklaces that also gives HP for Example
Exquisite Green Gem Pendant ( 13% HP Given)
Exquisite Dragon Eye Pendant (23% HP Given)
Exquisite BlueWater Drop Pendant (33% HP Given)
Tears Of Exquisite Goddesses (43% HP Given)
Guardianship Of Exquisite (53% HP Given)
Im not sure how much hp you get each level but you do get hp though..