So this is kind of two questions. Let me answer them backwards.
The boy-next-door (because I don't know if you know, bear with me if you do) is basically this character archetype in romance stories, or more broadly, a certain archetype of masculinity. A "boy next door" is a boy who you could meet in your neighborhood and fall in love with--he's handsome, but he's also sweet, and gentle, and helpful, and your parents love him, and he's innocent, and he's attainable. The boy next door is kind of the opposite of the "bad boy" archetype (and the gender-bent version of the "girl next door" archetype).
So Jinu plays boy-next-door in public as his "archetype" in the Saja Boys, in the same way that Abby plays the archetype of someone very sexual, or Mystery plays the archetype of an alluring mysterious man, and so on.
It's in the way he dresses, for one. Like I've said, he wears his hair very neat, but beyond that, I'd say that he's really the most toned-down dresser of the Saja Boys. His outfits, compared to the others, aren't done with flashy vivid colors, and they don't really have any flamboyant touches like the others have. His nails aren't even painted. His hair isn't dyed.
You can kind of imagine, like, if you were a teenager with strict parents, and you were a fan of the Saja Boys, they would approve of Jinu the most. Probably point him out and say "See, that one looks normal. He looks like a nice sweet boy. Why don't the others look like that?"
...That's the role that he's filling, for people like that who want something more traditional and familiar and not edgy in the slightest. For people who want a "nice young man."
You see this in his behavior as well, though there isn't much to point out given that A) we don't see Jinu talk that much as an idol, all things considered, and B) the Saja Boys all kind of move like a unit, so there's not much contrast of him against the other boys. They're all polite, publicly. But he's always polite as an idol, always staying very professional and poised.
It's like the girls in the Play Games With Us! audience say: he's "So hot! And respectful." ...And that's the role he's playing. Boy next door.
As to your other question. I think (correct me if I'm wrong) you're referring to a thing I've talked about a few times, which is Jinu's tendency to keep up his human disguise even when everybody present knows he's a demon. For instance, in most of his meetings with Gwi-Ma, he keeps up his human glamour. He does this in his meetings with Rumi as well, and he even does this when he's entirely by himself, like in this screenshot I posted again the other day.
^This is him standing in an empty hallway. Human disguise still intact.
...or when he's walking by himself on a completely dead street.
Still keeping up a human glamour.
Why? I think he likes being able to pretend he's human for a little while. He likes not having "a constant reminder of his shame" (his words that he used to describe his patterns) on his skin constantly.
...He likes the persona he's playing, essentially.
