At the beginning of a run, each player has to choose one class between 10 available. They are split into 3 categories :
Base health : 4750 /3750 / 3000 depending on difficulty. (Tanks +1000 ; Ranger -250)
Base mana : 200.
Melee classes have slightly higher movement speed.
It is not recommended to pick the same class multiple times, as they will share the same weaknesses and will often counter-synergize themselves (buffs not stacking etc.).
Each class has 6 unique spells plus Mystic Guard as well as a melee or ranged basic attack. Melee classes can kill most weak minions with 2 basic attacks, however ranged classes deal reduced damage to minions and need at least 6 hits. Weak minions include : tree seeds, imps, gravelings.
During fights various status effects can get temporarily applied to your character. Mystic guard and other damage nullifiers won't prevent you from being affected by those, here are some basic ones that come back often :
(✗ prevented / ~ hindered / ✓ available)
Using Saint's Soul will kill you even with on, however it will not kill you if you use it with Ice mage's or 16px invulnerability up.