Ninjas are supposed to be elite assassins trained to kill and follow orders from childhood. And their childhood trauma is the source of all the drama.

Whenever you watch a movie about dogs or wolves you know they're going to died. And whenever you watch a movie about ninja BFFs you know their final test will be to kill each other.
A common convention is to pair up a cold-hearted kill with someone with an actual ice powers. Someone trained not to feel, vs someone who can't feel anything because they're numb.

Ironically "ice" is used to sooth pain. And maybe, behind the ninja's vacant expression is a heart that feels everything.