Hunter
Starting weapon:

Crossbow
Bonus: +10% damage
Class skill: Hunter's Focus — Every 30 s, +15% attack speed for 8 s.
A tracker from the northern moors. Lost his village to the creatures of the night and swore to leave none standing. Few words, good aim.
How to unlock: Available from the start.
Mage
Starting weapon:

Fireball
Bonus: -10% cooldown
Class skill: Arcane Surge — Every 30 s, +20% attack area for 8 s.
Expelled from the academy for studying forbidden fire. Now tests theories where no one complains: in the middle of the horde.
How to unlock: Available from the start.
Knight
Starting weapon:

Axe
Bonus: -2 damage taken
Class skill: Wall of Steel — Every 30 s, pushes back nearby enemies and takes -3 damage for 8 s.
Last of an order that protected the roads. The armor is dented, but it still holds against any blow.
How to unlock: Available from the start.
Tamer
Starting weapon:

Whip
Bonus: +20% max health
Class skill: Call of the Pack — Summons a wolf that fights at your side until defeated (brown, white, black or golden: the rarer, the stronger); 30 s after it falls, another comes. On summon, +10% speed for 6 s.
Grew up among circus beasts and learned that every monster backs down at a firm crack. Tough as tanned leather.
How to unlock: Beat the Howling Forest
Rogue
Starting weapon:

Kunais
Bonus: +15% move speed
Class skill: Shadow Step — Every 30 s, +20% speed and +20% damage for 6 s.
No one knows their real name. In, strike, and gone before the monster sees where the blade came from.
How to unlock: Beat the Storm Laboratory
Monk
Starting weapon:

Aura
Bonus: Recovers +0.5 health per second
Class skill: Meditation — Every 30 s, recovers 10% health and becomes untouchable for 2 s.
Guardian of a forgotten monastery. Their calm drives back the shadows, and their body mends with every breath.
How to unlock: Beat the Witch Swamp
Necromancer
Starting weapon:

Grimoire
Bonus: +15% effect duration
Class skill: Raise the Dead — Raises 1 to 3 skeletons or, more rarely, 1 or 2 zombies (stronger), who fight until defeated; 30 s after they fall, raises more. On summon, +5% damage for 8 s.
Studied death to learn what keeps monsters standing. Turns that dark knowledge against them, and makes no apologies for it.
How to unlock: Defeat the Lich King in the Frozen Crypt
Princess
Starting weapon:

Wand
Bonus: +10% luck
Class skill: Holy Seal — Every 30 s, creates a seal of light for 6 s: heals 1 health per second and pushes away monsters that enter it.
Guardian of the five seals that bind the Count. After seeing the ballroom in ruins, she traded the ball for a wand and set out to hunt her captor.
How to unlock: Defeat Dracula in the Throne Tower: on Normal she may join you and on Hard the chance is higher
Draven
Starting weapon:

Rapier
Bonus: +5% damage and 3% life steal (half vampire)
Class skill: Bloodthirst — Every 30 s, for 6 s: +25% damage and 15% of damage dealt returns as health.
Draven, half vampire, the Count's own son. His father killed his mother and controlled him through blood for years. Now free, he swore to end the eternal night.
How to unlock: Free him in the Castle's East Wing: on Hard he may break free
Viscount
Starting weapon:

Pistol
Bonus: +20% projectile speed
Class skill: Moon Beast — Every 90 s, becomes a werewolf for 45 s: the pistol gives way to claws that slash nearby monsters; +30% speed, +40% attack speed and +20% damage.
A nobleman cursed by the moon. While he can, he settles things with elegance and gunpowder; when the beast awakens, with claws.
How to unlock: Free him from the cage in the Howling Forest, on Hard or Nightmare