WMG / 3 Ninjas

Mori adopted one of the boy's parents.

The movie never openly acknowledges why Mori is Japanese while the rest of them are Caucasian, leading to a lot of people asking what the movie doesn't answer. It's likely that Mori had adopted one of their parents at a young age — either being a family friend who took in a child when they lost their parents or some other reason — and raised them.

As far as we know, he didn't train either of the parents and instead skipped to the grandkids. While calling it a "heritage" would be stretching the definition, Mori is teaching his grandkids the ways of ninjitsu because, in his old age, he wishes for his family's ancient ways to be taught by someone in the family who he can be sure won't use it for evil purposes like Snyder.

If he's their paternal grandfather, this could explain the father's reluctance to let his kids hang out with him. Not just because of Snyder, but he has seen first hand some of the downsides of being around Mori. After all, it's likely that Mori had made a lot of enemies in his lifetime and its likely that he could have seen Mori kill people. If he's their maternal-grandfather, he is probably warry because he is only familiar with the ninja as pop culture portrays them as.

• It's repeatedly mentioned in the film that he's their maternal grandfather.