Source: Steam Community (opens in a new tab)
Below are all unlockable titles so far.
Slime Killer
+1 Luck.
Skeleton King
+2 Attack.
The Beginning of the End
+1 Attack, Defense, Magic Attack, Magic Defense.
Thief
-1 Luck.
+25% Experience.
+100% Gold Drop.
Vampire
+1% Life Regen.
Labyrinth Boss
+2 Attack, Defense.
+50 Life.
Bull
+3 Defense.
Ability: Flame Slash.
Transgenesis
+10% Pharmacology (increases the effect of items).
Resistance: Poison.
Ability: Poison.
Forbidden Fruit
+1 Magic Attack, Magic Defense.
+ 20 Mana.
Ability: Forbidden Fruit.
Succubus
+20 Agility.
+5 Evasion.
Passive: Auto Attack.
Connoisseur of Chests
+5 Luck.
+100% Gold Drop.
Playing with Fire
+10 Magic Attack, Magic Defense.
+5 Attack, Defense.
+20% Fire Elemental Resistance.
+10% Magic Damage.
Lazy
-1 Attack, Defense, Magic Attack, Magic Defense, Agility, Luck, Health, Mana.
-1% Experience.
+1% Mana Cost.
Passive: Auto Attack.
Rat King
+10 Attack, Agility, Luck.
+20% Evasion.
Brother
+25% Reflect Spells, Counter.
+5% Life.
+5% Regeneration: Life, Mana, Faith.
+1 Turn.
Cherubim
+1 Attack, Defense, Magic Attack, Magic Defense, Agility, Luck.
+7 Life, Mana.
Ability: Double Attack, Purge.
Skeleton Mage (next update)
+15 Magic Attack.
+5 Attack.
+50 Life.
+50 Mana.
Amnesia
+20 Agility.
+10% Pharmacology.
Resistances: Confusion, Fascination, Drowsiness, Taken, Silence.
+10% Evasion, Critical, Spell Evasion, Counter.
Protector (next update)
+20 Attack, Magic Attack.
+200 Life.
+5% Regeneration: Life, Mana, Faith.
Giant Slayer
+20 Attack, Defense.
+200 Life.