You don't really play as Kendo (Satoshi's uncle) or as Satoshi, you are more like an outsider as sometimes you make decisions as Satoshi and sometimes you do it as Kendo.
(I will try to not spoil) It is a good NTR because in my opinion Satoshi doesn't deserve the girl or at least is not up to the task. He is, with his girlfriend, passive, indecisive and egoistical. You have the feeling the couple don't share much hobbies and interest in their respective hobbies. And the sad part is that the girl herself seems to try harder than Satoshi for their relationship. And Satoshi is kind of a dick with his Uncle. However Kendo seems like a genuine dude, he really wants to see his nephew work things out with his girlfriend. And he has much more in common with the girl than his nephew. He does everything to help them while having some kind of crush over the girl. I won't give you too much detail, but the story feels natural and the character develloment, mainly between Kendo and Mary is pretty good.