Here is Nastuo growing up, to the right is his younger self compared to the left.
Despite how he acts, he's very sentimental, he has a very dark past like NaNa.
(He's very old/immortal)

Because he can't die and eventually stops growing at one point he doesn't get attached to people, NaNa being the only person he can rely on since both are immortal and been siblings since they were kids.

I drew this when I was kind of sad 😛