Critical damage is the amount of damage done when a hyper lands a critical hit. Damage done is 150% attack damage (attack damage x 1.5). The damage percent can be boosted with equipment. Critical damage is not affected by Critical Rate.
Damage can be boosted to 70% with the item, Dragon's Bane.
Turner naturally has 15% Critical Rate.
Sapitelbub is capable of reaching the highest extra damage of 100% with his unique item Bloodlust Band, Dragon's Bane and Slayer's Crossbow equipped.