You cannot get angered very easily. It takes nothing short of a close friend, family member, or loved one dying in front of you to make you truly enraged. When you're not angry (which is virtually every moment of your lifetime), you're one of the most caring and lovable people in the world. Because of this you'll virtually never kill except in the most dire of circumstances.

Under normal circumstances this gives you +2 to defensive rolls, except for Genjutsu Dodge. However, when you're actively trying to save somebody's life this is replaced with a +6 bonus to all defensive rolls, +2 to your Chakra, Stamina, and Willpower multipliers, along with +10% to all your damage rolls. It's possible to have other conditions (typically harder to meet) if you talk with a GM.
Please note that the effects of this unique come largely from the person's outlook on life; a cruel or aggressive person obviously wouldn't have a "Gentle Soul".