Hi! So Ninjutsu and Taijutsu are usually the base value which can be affected by secondary stats like Defense, Resistance, Injury, Critical, Damage Reduction, Ninjutsu Pen3tration and Taijutsu Pen3tration, etc. This thread goes over more in-depth in regards to secondary stats.

Just like what was said above; Critical is a value that gives you a chance in performing a Critical attack. Critical is two parts; it both increases the chance of Critical, as well as decreases the chance of receiving a Critical. This means that your own ninja's Critical vs an opponent's ninja's Critical will play a crucial part on whether your ninjas will perform a Critical.

Injury on the other hand affects the damage applied from Critical as well as decreases the Critical damage taken from an enemy. What that means is that your own ninja's Injury vs an opponent's ninja's Injury will also play a crucial role on how much damage a Critical will apply.

Which is why these two stats play hand in hand as it affects both your chance of performing and Critical and also how much damage (Injury) you will receive from the Critical.