The Mood System allows you to increase the base stat of a ninja enormous for sacrificing some stats in an other category.
To use it, you have to first use Meatballs (10 coupons each) to change the mood of the ninja of your choice. Afterwards you will see that one stat is highlighted red and the other green, you can keep using Meatballs to refresh the mood and change the stats. The red stat indicates that you will loose that amount of Ninjutsu, Attack, Life, Defense or Resistance. The green stat indicates how much you gain. As example as Azure Fang most of my skills scale with Ninjutsu, so I decided to sacrifice some Attack to increase the Ninjutsu damage output.
Once you have the stat you like to increase you can use Mood Scrolls (15 coupon each) which can be used to level up your mood. Till level 5 the cost for each level are 2 Scrolls each and will give up to 25% in the positive stat giving an increase of 25% for just 150 coupons. The cost increases past level 5 making it harder to level up and increasing the negative amount of stat you get later on.